BERLIN — German Chancellor Angela Merkel told Russian President Vladimir Putin during a telephone conversation on Sunday a referendum planned for March 16 on Crimea joining Russia violated Ukraine's constitution and was against international law.

She also regretted that there had not been any progress on forming an "international contact group" which could find a political solution to the crisis in Ukraine, the German government said in a written statement. "She pointed out the urgency of finally coming to a substantial result on this."

On Thursday, Merkel said if no international contact group was formed in the coming days and no progress was made in negotiations with Russia, it was possible the European Union would impose on Russia further sanctions like travel restrictions and freezing financial accounts.