Russian President Vladimir Putin agreed in a conference call with the leaders of France, Germany and Ukraineto use his influence on separatists to start an agreed prisoner swap with Ukraine, a German government spokesman said on Thursday.

The four leaders agreed to implement last week's Minsk agreement on a ceasefire in Ukrainedespite serious breaches, said spokesman Steffen Seibert, echoing a statement made byFrance.

"The prisoner exchange must also start. President Putin agreed to influence the separatists in this direction," said Seibert in a statement.