"We are waiting for (Russian President Vladimir) Putin's agreement and we agree that if there is no real progress this week, we want to work together at European level for a significant tightening of sanctions," Merz said at a news conference with his Greek counterpart.

Germany and its allies will move to confiscate frozen Russian assets if legally possible, Chancellor Friedrich Merz said in an interview with the Zeit newspaper published on Thursday.

The ultimatum from Berlin comes as Ukraine said Russia was still carrying out attacks along the front line, despite a ceasefire proposal agreed on the weekend among Kyiv's allies for a 30-day ceasefire to start Monday. The Kremlin said such an ultimatum "is unacceptable for Russia."