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.

Air raid warnings sounded throughout Ukraine for a second straight morning as officials advised residents to conserve energy and stock up on water. The strikes have knocked out power across the country 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 capital's streets. "We already got used to this. And we will keep fighting."

The leaders of the Group of Seven industrial powers condemned the bombardment and said they would "stand firmly with Ukraine for as long as it takes." Their pledge defied Russian warnings that Western assistance would prolong the war and the pain of Ukraine's people.

Russia launched the widespread attacks in retaliation for a weekend explosion that damaged the Kerch Bridge between Russia and the Crimean Peninsula, which Moscow annexed in 2014. Russian President Vladimir Putin alleged that Ukrainian special services masterminded the blast. The Ukrainian government has applauded it but not claimed responsibility.

Ukrainian President Volodymyr Zelenskyy told the G-7 leaders during a virtual meeting that during the past two days Russia fired more than 100 missiles and dozens of drones at Ukraine, and that while Ukraine shot down many of them, it needs "more modern and effective" air defense systems.

The Pentagon earlier announced plans to deliver the first two advanced NASAMs anti-aircraft systems to Ukraine in the coming weeks. The systems, which Kyiv has long wanted, will provide medium- to long-range defense against missile attacks.

In a phone call with Zelenskyy on Tuesday, President Joe Biden "pledged to continue providing Ukraine with the support needed to defend itself, including advanced air defense systems," the White House said.

Zelenskyy thanked the U.S. and also Germany for speeding up the delivery of the first of four promised IRIS-T air defense systems. Ukraine's defense minister tweeted that the German system had just arrived, and that a "new era" of air defense for Ukraine had begun.

Zelenskyy also urged the G-7 leaders to respond "symmetrically" to the attacks on the Ukrainian energy sector by doing more to stop Russia from profiting off its exports of oil and gas.

"Such steps can bring peace closer," he said. "They will encourage the terrorist state to think about peace, about the unprofitability of war."

Ukrainian officials said the 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 tougher action in Ukraine and criticized the Russian military for a series of embarrassing battlefield setbacks.

Pro-Kremlin pundits lauded the attacks as an appropriate response to Kyiv's successful counteroffensives. Many of them argued that Moscow should keep up the intensity to win a war now in its eighth month.

The head of Britain's cyber-intelligence agency, Jeremy Fleming, said Tuesday in a rare public speech that Russia is running out of military supplies and struggling to fill its ranks.

"Russia's forces are exhausted," Fleming said. "The use of prisoners as reinforcements, and now the mobilization of tens of thousands of inexperienced conscripts, speaks of a desperate situation."

Like Monday's strikes, the bombardment Tuesday struck both energy infrastructure and civilian areas. One person was killed when 12 missiles slammed into 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 Vinnytsia regions also took hits. Officials said Ukrainian forces shot down an inbound Russian missile before it reached Kyiv, but the capital region experienced rolling power outages as a result of the previous day's strikes.

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.

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 in Geneva. "Attacks targeting civilians and objects indispensable to the survival of civilians are prohibited under international humanitarian law."

War crimes investigations have long been underway in towns where mass graves were found, along with other evidence of atrocities, after they were liberated from Russian occupation. In Lyman, a city in the eastern Donetsk region, forensic workers pulled several bodies from a mass grave Tuesday, part of an arduous effort to piece together evidence of what happened under more than four months of Russian occupation. Regional Gov. Pavlo Kyrylenko said the bodies of 32 Ukrainian soldiers have been exhumed so far from one mass grave.

The tempo of the war in the last month fanned concerns that Moscow might broaden the battlefield and resort to using nuclear weapons in Ukraine. As Ukraine's counteroffensives in the east and south forced Russia's troops to retreat from some areas, a cornered Kremlin ratcheted up Cold War-era rhetoric.

Russian Foreign Minister Sergey Lavrov said Tuesday that Moscow would only employ nuclear weapons 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 alliance would hold annual war exercises testing the state of readiness of its nuclear capabilities next week as scheduled.

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

Stoltenberg called Putin's rhetoric "irresponsible" but said he believes "Russia knows that a nuclear war can never be won and must never be fought."

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

The G-7, leaders who held the emergency meeting in response to Monday's attack, said the "indiscriminate attacks on innocent civilian populations constitute a war crime" and reaffirmed their "commitment to providing the support Ukraine needs to uphold its sovereignty and territorial integrity."

The pledge appeared to come in response to Kremlin warnings that Western military assistance, including training Ukrainian soldiers in NATO countries and feeding real-time satellite data to target Russian forces, increasingly made Ukraine's allies parties to the conflict.

Kremlin spokesman Dmitry Peskov said continued U.S. weapons deliveries to Ukraine would prolong the fighting and inflict more damage on the country without changing Russia's objectives.

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.

Russians previously detained the facility's general director and released him following pressure from International Atomic Energy Agency Director General Rafael Mariano Grossi.

Grossi met with Putin on Tuesday in St. Petersburg and urged him to agree to a "safety and security protection zone" around the occupied plant to prevent a radiation disaster.

From The Associated Press