Ukrainian President Volodymyr Zelensky on Sunday said that Ukraine and Russia would conduct the first diplomatic talks since the Kremlin-launched invasion, with delegations from both countries meeting at the Ukrainian border with Belarus. The two countries would meet near the Pripyat River “without preconditions,” Zelensky’s office said in a Telegram message.

But tensions continued to escalate, with Russian President Vladimir Putin stating Sunday that he had put his nuclear deterrence forces into high alert, attributing the move to “aggressive statements” from the West against Russia. The White House called the order an example of “manufacturing threats that don’t exist.”

The European Union, meanwhile, announced it will shut down airspace to Russian planes and finance weapons purchases to Ukraine — strict measures that come as several nations, including the U.S., vow to block the Kremlin’s access to its sizable foreign currency reserves in the West and to cut off some Russian banks from the SWIFT financial messaging system.

Earlier Sunday, Russian forces pushed into Kharkiv, sparking a battle for control in Ukraine’s second-largest city. By afternoon local time, the city was quieter, with the sounds of bombardment fading from downtown and Kharkiv’s governor announcing the city remained under government control.

Over four days of fighting, the United Nations’ refugee agency said Sunday that 368,000 people have fled Ukraine. In a sign of how the war is quickly upending Europe’s status quo, German Chancellor Olaf Scholz announced a major boost in defense spending, saying it was time to “invest significantly more” in security and protecting democracy.