United Nations Secretary-General Antonio Guterres on Monday sounded the alarm over Russia raising the alert level for its nuclear forces after invading Ukraine, describing it a “bone-chilling development.”

“The prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility,” Guterres told reporters, and repeated his call for an immediate cessation of hostilities.

Russia’s invasion of Ukraine that began on Feb. 24 has so far sent more than 2.8 million people fleeing across Ukraine’s borders and trapped hundreds of thousands in besieged cities while triggering broad Western sanctions on Russia.

Russian President Vladimir Putin late last month said that his nation’s nuclear forces should be put on high alert, raising fears that Russia’s invasion of Ukraine could lead to nuclear war. U.S. officials have said they have seen no reason so far to change Washington’s nuclear alert levels.

Guterres has also called for the preservation of the security and safety of nuclear facilities after a fire at the Zaporizhzhia nuclear power plant in Ukraine, Europe’s biggest of its kind, that broke out during a takeover of the plant by Russian forces.

“It is time to stop the horror unleashed on the people of Ukraine and get on the path of diplomacy and peace,” Guterres said.

He also said the U.N. was going to allocate a further $40 million from its Central Emergency Response fund to ramp up humanitarian assistance for Ukraine.

“This funding will help get critical supplies of food, water, medicines and other lifesaving aid into the country as well as provide cash assistance,” Guterres said.

Reuters