The French and German leaders spoke for more than two hours by telephone with Ukrainian President Petro Poroshenko and Russia’s Vladimir Putin in a effort to seek progress ahead of the expiration of a ceasefire, the French president’s office said.

Francois Hollande and German Chancellor Angela Merkel "stressed the importance of further concrete progress towards the stabilisation of security on the ground, the extension of the ceasefire and the implementation of the peace plan presented by the Ukrainian authorities," Mr Hollande's office said in a statement.

Ukraine extended its truce by 72 hours until Monday night, coinciding with a deadline set by European Union leaders on Friday for pro-Russian rebels to agree ceasefire verification arrangements, return border checkpoints to Kiev authorities, free hostages and launch serious talks on implementing Mr Poroshenko's peace plan.

The separatist rebels in eastern Ukraine on Saturday released a second group of four monitors from the Organization for Security and Cooperation in Europe who had been seized on 29 May, a Reuters witness said.

However the ceasefire appeared under threat when three members of the Ukrainian military were killed in a rebel attack on their post near the eastern city of Slaviansk.