The United States and its allies heavily criticized Russia on Friday at the United Nations over its shelling and seizure overnight in Ukraine of Europe's biggest nuclear power plant, and some demanded that Moscow not let such an attack happen again.

Many of the Security Council's 15 envoys expressed "grave concern" and shock, warning against the possibility of a repeat of the 1986 Chernobyl disaster - a nuclear accident in Ukraine when it was part of then Soviet Union considered to be the worst in history.

They said the attack was against international humanitarian law and urged Moscow to refrain from any military operations targeting the nuclear facilities and allow Ukrainian personnel to be allowed onto the plant to carry out their work.

"The world narrowly averted a nuclear catastrophe last night," Linda Thomas-Greenfield, U.S. Ambassador to the United Nations, told an emergency meeting of the Security Council, convened following the seizure of the Zaporizhzhia nuclear power plant in southeastern Ukraine by Russian troops.

"Russia’s attack last night put Europe’s largest nuclear power plant at grave risk. It was incredibly reckless and dangerous. And it threatened the safety of civilians across Russia, Ukraine and Europe," Thomas-Greenfield said.

As shells hit the area early on Friday, a blaze broke out in a training building - triggering a spasm of alarm around the world before the fire was extinguished and officials said the facility was safe.

Ukraine ambassador to the U.N. Sergiy Kyslytsya called for all Russian forces to be withdrawn from the plant and a no-fly zone over the country to protect the civilian population from air attacks.

Officials remained worried about the precarious circumstances, with Ukrainian staff operating under Russian control in battlefield conditions beyond the reach of administrators.

"France strongly condemns this attack on the integrity of a nuclear structure, which we need to guarantee," Nicolas de Riviere said in his speech. "The results of the aggression of Russia against Ukraine are possibly devastating for human health and the environment," he added.

United Kingdom ambassador to the United Nations Barbara Woodward said: "It must not happen again. Even in the midst of an illegal invasion of Ukraine, Russia must keep fighting away from and protect the safety and security of nuclear sites."

International Atomic Energy Agency chief Raphael Grossi described the situation as "normal operations, but in fact there is nothing normal about this."

Thousands of people are believed to have been killed or wounded and more than 1 million refugees have fled Ukraine since Russian began its invasion on Feb. 24. Western nations retaliated with sanctions that have plunged Russia into economic isolation.

Russia's envoy to the United Nations Vassily Nebenzia dismissed Western uproar over the nuclear power plant and called Friday's Security Council meeting another attempt by Ukrainian authorities to create "artificial hysteria".

"At present, the Zaporizhzhia nuclear power plant and adjacent territory are being guarded by Russian troops," he said.

Separately, France and Mexico are working on a resolution to the U.N. Security Council next week that will address the humanitarian impact of Russia's invasion, diplomats said.

(Reporting by Humeyra Pamuk and Doina Chiacu in Washington Editing by Grant McCool)