NATO leaders will not issue an invitation for Ukraine to join the alliance at a summit in Vilnius in mid-July, NATO Secretary-General Jens Stoltenberg said on Monday.

"At the Vilnius summit and in the preparations for the summit, we are not discussing to issue a formal invitation," he told reporters after meeting German Chancellor Olaf Scholz in Berlin, adding leaders would talk about how to move Ukraine closer to NATO.

At the same time, Stoltenberg warned against accepting a frozen conflict in Ukraine in return for an end to the war.

"We all want this war to end, but a just peace cannot mean freezing the conflict and accepting a deal dictated by Russia," he said.