Russian forces showered Ukraine with more missiles and munition-carrying drones Tuesday after widespread strikes killed at least 19 people in an attack the U.N. human rights office described as “particularly shocking” and amounting to potential war crimes.

In a statement released Tuesday, the leaders of the Group of Seven nations condemned the recent attacks "in the strongest possible terms" and pledged to hold Russian President Vladimir Putin and all responsible to account.

Air raid warnings sounded throughout the country for a second straight morning as Ukrainian officials advised residents to conserve energy and stock up on water. Strikes in the capital and 12 other regions Monday caused widespread power outages and pierced the relative calm that had returned to Kyiv and many other cities far from the war’s front lines.

“It brings anger, not fear,” Kyiv resident Volodymyr Vasylenko, 67, said as crews worked to restore traffic lights and clear debris from the city’s streets. “We already got used to this. And we will keep fighting.”

Russia launched the widespread attacks in retaliation for a weekend explosion that damaged a bridge linking the country to the Crimean Peninsula, which Moscow annexed from Ukraine in 2014. Russian President Vladimir Putin alleged the Ukrainian special services masterminded the Saturday attack on the Kerch Bridge.

The leaders of the Group of Seven nations – U.S. President Joe Biden, Canadian Prime Minister Justin Trudeau, French President Emmanuel Macron, German Chancellor Olaf Scholz, Italian Prime Minister Mario Draghi, Japan's Prime Minister Fumio Kishida, United Kingdom Prime Minister Liz Truss and European Union leaders Ursula von der Leyen and Charles Michel – held a virtual meeting to discuss discuss their commitment to supporting Ukraine amid the ongoing conflict. 

"Our meeting took place against the backdrop of the most recent missile attacks against civilian infrastructure and cities across Ukraine, leading to the death of innocent civilians," a statement from the G7 leaders reads. "We condemn these attacks in the strongest possible terms and recall that indiscriminate attacks on innocent civilian populations constitute a war crime. We will hold President Putin and those responsible to account."

The leaders rejected Russia's attempted annexation of four regions in Ukraine and pledged that "we will never recognise this illegal annexation or the sham referenda that Russia uses to justify it."

Ukrainian President Volodymyr Zelenskyy participated in the meeting, urging the G-7 leaders during a virtual meeting Tuesday to hit Russia’s energy sector with tougher sanctions.

"There can be no dialogue with this leader of Russia, who has no future," Zelenskyy said, adding: "Now, one person is blocking peace – and this person is in Moscow."

The group said that they "reassured President Zelenskyy that we are undeterred and steadfast in our commitment to providing the support Ukraine needs to uphold its sovereignty and territorial integrity."

"We will continue to provide financial, humanitarian, military, diplomatic and legal support and will stand firmly with Ukraine for as long as it takes," the leaders pledged. "We are committed to supporting Ukraine in meeting its winter preparedness needs."

Russian President Vladimir Putin said the strikes were in retaliation for what he called Kyiv’s “terrorist” actions — a reference to Ukraine’s attempts to repel Moscow’s invasion forces, including an attack Saturday on a key bridge between Russia and the annexed Crimean Peninsula that Putin called a “terrorist act” masterminded by Ukrainian special services.

Putin vowed a “tough” and “proportionate” response should Ukraine carry out further attacks that threaten Russia’s security. “No one should have any doubts about it."

But one expert told Spectrum News that Putin's attacks "really demonstrate his weakness right now" in the monthslong conflict.

"I think it's pretty clear that Putin is losing the conflict," said Robert Orttung, a research professor at Elliott School of International Affairs at George Washington University. "And Ukrainians are demonstrating greater and greater capacity to attack them to attack their supply lines to inflict real military damage under troops."

In their statement, the G7 leaders accused Russia of "blatantly" violating the principles of the United Nations charter – "They cannot and do not give Russia a legitimate basis to change Ukraine’s borders," they wrote – and called on all nations to demand that Russia end the conflict with Ukraine.

"We deplore deliberate Russian escalatory steps, including the partial mobilisation of reservists and irresponsible nuclear rhetoric, which is putting global peace and security at risk," the G7 leaders wrote. "We reaffirm that any use of chemical, biological or nuclear weapons by Russia would be met with severe consequences."

The leaders also called Russia's actions at Ukraine's Zaporizhzhya Nuclear Power Plant, Europe's largest such plant, as an "irresponsible escalation" .

As Russian forces pounded three districts around the Zaporizhzhia Nuclear Power Plant overnight, Ukraine’s state nuclear operator said Russian forces kidnapped the plant’s deputy human resources director, Valeriy Martyniuk.

Russians previously detained plant General Director Ihor Murashow and released him following pressure from International Atomic Energy Agency Director General Rafael Mariano Grossi.

Grossi, who met with Putin in St. Petersburg on Tuesday, urged the Russian leader to agree to establish a “safety and security protection zone” around the Russian-occupied plant to prevent shelling at and near the site from causing a radiation disaster.

“Now, more than ever, during these extremely difficult times, a protection zone must be established around the ZNPP. We can’t afford to lose any more time,” Grossi said in a statement.

They also called on Belarus to cease its support for Russia's invasion of Ukraine.

"The announcement of a joint military group with Russia constitutes the most recent example of the Belarusian regime’s complicity with Russia," they wrote. "We renew our call on the Lukashenko regime to fully abide by its obligations under international law."

They added that they were "deeply troubled by the deliberate damage" to the Nord Stream offshore gas pipelines between Russia and Germany and "welcome ongoing investigations" into the incident.

Ukrainian officials said the previous day’s diffuse strikes on power plants and civilian areas made no “practical military sense.” However, Putin’s supporters had urged the Kremlin for weeks to take more drastic steps in Ukraine and actively criticized the Russian military for a series of embarrassing battlefield setbacks.

Pro-Kremlin pundits lauded Monday’s attack as an appropriate and long-awaited response to Kyiv’s recently successful counteroffensives, and many of them argued that Moscow should keep up the intensity in order to win a war now in its eighth month.

The bombardment Tuesday struck both energy infrastructure and civilian areas, just as Monday’s attacks did. One person was killed when 12 missiles slammed into public facilities in the southern city of Zaporizhzhia, setting off a large fire, the State Emergency Service said. A local official said the missiles hit a school, residential buildings and medical facilities.

Energy facilities in the western Lviv and Vinnitsya regions also took hits. Although officials said Ukrainian forces shot down an inbound Russian missile before it reached Kyiv, the capital region experienced rolling power outages as a result of the previous day’s deadly strikes.

The governor of the Mykolaiv region, Vitaliy Kim, urged residents to remain in bomb shelters as “there are enough missiles still in the air.”

The State Emergency Service said 19 people died and 105 people were wounded in Monday’s strikes. At least five of the victims were in Kyiv, Mayor Vitali Klitschko said. More than 300 cities and towns lost power, from the capital to Lviv on the border with Poland.

Beside the usual sirens, a new type of loud alarm that blared automatically from mobile phones jolted Kyiv residents early Tuesday. A text message warning of the possibility of missile strikes accompanied the caustic-sounding alert.

A spokesperson for the office of the U.N. High Commissioner for Human Rights said Tuesday that strikes on “civilian objects,” including infrastructure such as power plants, could qualify as a war crime.

“Damage to key power stations and lines ahead of the upcoming winter raises further concerns for the protection of civilians and in particular the impact on vulnerable populations,” Ravina Shamdasani told reporters at a U.N. briefing in Geneva. “Attacks targeting civilians and objects indispensable to the survival of civilians are prohibited under international humanitarian law.”

As Ukrainian forces grew increasingly bold following a series of counteroffensive successes, a cornered Kremlin ratcheted up Cold War-era rhetoric in the last month and fanned concerns it might broaden the war and use nuclear weapons in Ukraine.

Russian Foreign Minister Sergey Lavrov addressed the issue Tuesday, saying Moscow would only resort to that if the Russian state faced imminent destruction. Speaking on state TV, he accused the West of encouraging false speculation about the Kremlin’s intentions.

Russia’s nuclear doctrine envisions “exclusively retaliatory measures intended to prevent the destruction of the Russian Federation as a result of direct nuclear strikes or the use of other weapons that raise the threat for the very existence of the Russian state,” Lavrov said.

In Brussels, NATO Secretary-General Jens Stoltenberg said the 30-nation military alliance would hold long-planned exercises next week to test the state of readiness of its nuclear capabilities.

The exercise, dubbed “Steadfast Noon,” is held annually. It involves fighter jets capable of carrying nuclear warheads but does not any live bombs. Conventional jets, and surveillance and refueling aircraft routinely take part.

Asked whether it was the wrong time for such an exercise, Stoltenberg replied: “It would send a very wrong signal now, if we suddenly cancelled a routine, long-time planned exercise because of the war in Ukraine.”

Stoltenberg said Russian President Vladimir Putin’s nuclear rhetoric over the war in Ukraine is “irresponsible,” and he said that “Russia knows that a nuclear war can never be won and must never be fought.”

NATO as an organization does not possess any nuclear weapons. They remain under the control of three member countries – the United States, the U.K. and France.