Ukrainian President Volodymyr Zelenskyy on Wednesday warned Russia was using nuclear power plants to "blackmail" the world, speaking on the anniversary of the Chernobyl disaster.

"Thirty-seven years ago the Chernobyl nuclear power station disaster left a giant scar on the whole world," Zelenskyy wrote on Telegram.

"Last year, the occupying power did not just invade this power station. It again put the world in danger" of another disaster, he said.

"We have to do everything to prevent the terrorist state from using nuclear power stations to blackmail Ukraine and the world."

A reactor at Chernobyl, located around 60 miles north of Kyiv, exploded on April 26, 1986, when Ukraine was still part of the Soviet Union.

The incident, which is considered the world's worst-ever nuclear disaster, contaminated vast areas of Ukraine, Belarus and Russia. Swathes of western Europe were also exposed to radiation.

On the first day of Russia's invasion on February 24, 2022, Russian troops took over Chernobyl after entering from Belarus. 

The site has not been in activity since 2000.

Russian forces spent the next month at the power plant before withdrawing. Ukraine accused them of looting and exposing themselves to radiation by digging trenches inside the exclusion zone.

Kyiv has strengthened defenses along its northern border in case of another attack.

In southern Ukraine, Russian forces have taken control of the Zaporizhzhia nuclear power plant — Europe's largest — which previously accounted for 20% of Ukraine's electricity production.

The Director General of the International Atomic Energy Agency (IAEA), Rafael Mariano Grossi, noted Wednesday that 8.4 million people were exposed to radiation in the Chernobyl disaster, and he vowed that he would work to avoid the same fate for Zaporizhzhia. Grossi said he spoke with Zelenskyy on Wednesday to mark the Chernobyl anniversary.

The United Nations stepped in early on the nuclear cleanup, rehabilitation, environment and health of the Chernobyl disaster, when the then-Soviet Union "acknowledged the need for international" help, four years after the radioactive cloud spread over Ukraine, Russia and Belarus.

The Zaporizhzhia plant, which has been in and out of Russian control since the start of the war over a year ago, is under the supervision of the IAEA. Grossi said that when the situation "took a very serious turn" last year at Zaporizhzhia, the emergency unit of the IAEA went into the "highest level of alert" for the first time since the 2011 Fukushima Daiichi accident.

The power station continued functioning in the first months of the invasion despite fighting around it, but was stopped in September.

Kyiv and Moscow have accused each other of shelling the site in strikes that have raised fears of a repeat of the Chernobyl disaster.

Last month, Group of Seven (G7) foreign ministers meeting in Japan, including U.S. Secretary of State Antony Blinken, issued a statement condemning Russia for announcing a plan to station "tactical nuclear weapons" in Belarus, just across Ukraine's northern border. It was the first time Russia has threatened to deploy nuclear weapons in another country since the Cold War. The G7 includes the United States, Canada, the United Kingdom, Italy, Germany, France and Japan.

"Russia's irresponsible nuclear rhetoric and its threat to deploy nuclear weapons in Belarus are unacceptable," the G7 said in a statement. "Any use of chemical, biological or nuclear weapons by Russia would be met with severe consequences."

CBS News' Pamela Falk contributed to this report.

In:
• Belarus
• Chernobyl
• Nuclear Power Plant
• Ukraine
• Russia
• Volodymyr Zelenskyy