Powerful explosions shook the area around Ukraine's Zaporizhzhia Nuclear Power Plant Saturday evening and again Sunday morning, the U.N. said, abruptly ending a period of relative calm at the facility.

The International Atomic Energy Agency team at Zaporizhzhia said there was damage to some buildings, systems and equipment at the plant, but none threatened nuclear safety and security. There have been no casualty reports.

IAEA Director-General Rafael Mariano Grossi said in a statement Sunday that the explosions further underlined the urgent need for measures to help prevent a nuclear accident there.

"The news from our team yesterday and this morning is extremely disturbing," Grossi said. "Explosions occurred at the site of this major nuclear power plant, which is completely unacceptable. Whoever is behind this, it must stop immediately. As I have said many times before, you're playing with fire!" he added.

The director-general renewed his urgent appeal to both sides in the conflict to agree and implement a nuclear safety and security zone around the ZNPP as soon as possible. In recent months, he has engaged in intense consultations with Ukraine and Russia about establishing such a zone, but so far without an agreement.

Meanwhile, Britain's Defense Ministry said in its daily intelligence update Sunday that Russia's recent withdrawal from Kherson "was conducted in relatively good order" and its success "is likely partially due to a more effective, single operational command under General Sergei Surovikin."

The ministry said Russian vehicle losses were likely in the tens rather than the hundreds, while any left behind equipment was "successfully destroyed by Russian forces to deny it to Ukraine."

The report warned, however, the Russian force "remains riven by poor junior and mid-level leadership and cover-up culture."

Saturday, U.S. Defense Secretary Lloyd Austin said Ukraine's defense has global implications. At the Halifax International Security Forum in Canada, he warned that Russia's invasion of Ukraine offers a preview of a world where nuclear-armed countries could threaten other nations.

'Putin's fellow autocrats are watching,' Austin said. 'And they could well conclude that getting nuclear weapons would give them a hunting license of their own. And that could drive a dangerous spiral of nuclear proliferation,' he added.

Austin also said Moscow's efforts to gain support from countries such as Iran and North Korea create new security challenges for the United States and its allies.

Earlier, the Pentagon's top policy adviser, Colin Kahl, said Russia is trying to deplete Ukraine's air defenses and achieve dominance over Ukrainian skies.

Russia has been pummeling Ukraine with missile strikes throughout the past week, the heaviest wave since Moscow invaded nine months ago.

In Kyiv people woke up Saturday to several inches of snow. Ukrainian authorities in the capital are warning of a 'complete shutdown,' as subzero temperatures grip the country.

Russian airstrikes have inflicted heavy damage on the energy grid of the Ukrainian capital while they continue to pound Ukraine - from Kyiv in the north to Odesa in the south - crushing almost half of Ukraine's energy system, Ukrainian Prime Minister Denys Shmyhal said.

'They are determined to destroy our power grids,' said Andriy Yermak, head of the Office of the President of Ukraine, in a speech at the Halifax forum.

'The calculation is simple: a humanitarian catastrophe. Moscow always considers frost and darkness as its allies. It always uses the deprivation of basic life needs as a war tool. It always despises humanitarian law. Russia is a terrorist state,' Yermak said.

Amid freezing temperatures, difficulties with energy supplies persist throughout Ukraine - Ukrainian President Volodymyr Zelenskyy said Saturday, in his nightly video address.

'Energy workers are doing everything possible to give people a normal life,' he said.

He added transport connections are being restored in Kherson.

'There is the first train from Kyiv. We create new opportunities for people every day,' Zelenskyy said.

In message earlier Saturday, Zelenskyy also addressed the annual Halifax meeting.

'The end of the war doesn't guarantee peace. Russia is now looking for a short truce, a respite to regain strength... such a respite will only worsen the situation,' he said.

Some information for this report came from Agence France-Presse, Reuters and The Associated Press.