A nuclear conflict over Ukraine is a real and dire possibility if the war further escalates between Russia and the West, warned United Nations Secretary General Antonio Guterres on Monday.

In a series of remarks to reporters about the possibility of nuclear war over Ukraine, Guterres sounded the alarm that the frightening possibility is no longer as improbable as it once was in the aftermath of the Cold War.

“The prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility,” Guterres said as he continued to push for peace in Europe.

This stark admission by the UN chief comes after a series of incidents that raised the possibility of a nuclear incident in the weeks since Russia launched its invasion of Ukraine on Feb. 24.

Last month, Russian President Vladimir Putin ordered his nuclear forces to go on high alert in response to the “aggressive statements” aimed at Russia following his decision to attack Ukraine. While the U.S. intelligence community cautioned last week that the declaration was likely aimed at achieving deterrence, outside experts warn that there is a real danger Putin may resort to a tactical nuclear strike if he feels cornered.

The Biden administration has been measured in its response to Putin’s declaration. Officials have been cautious about cornering Putin or feeding his fears that Washington is seeking regime change in Russia. At the same time, President Joe Biden has used the specter of nuclear war to dissuade talk of a U.S.-Russia military clash, warning last week that this would spark "World War III."

But Guterres was also mindful that the nuclear risk stemming from the war in Ukraine does not only come from a conscious choice to deploy a nuke. Instead, the secretary general drew attention to the danger that an accidental strike on a Ukrainian nuclear plant could have for the world, urging both sides to ensure the safety of these facilities.

Since invading Ukraine, Russia has sparked fears of nuclear disaster twice.

During the early days of the war, Russian forces attacked and took control of Chernobyl, site of the 1986 disaster that sent an atomic cloud across Europe. On March 4, Russian forces shelled Ukrainian positions near the Zaporozhye nuclear power plant, the largest in Europe, before capturing it.

According to the International Atomic Energy Agency (IAEA), Russia informed it that Ukrainian employees were left in control of both plants and that it would deploy specialists to help ensure the plants are safely operated.