US Secretary of State John Kerry has urged Russian President Vladimir Putin to publicly call on separatists in Ukraine to lay down their arms.

He said Russia would face "greater costs" unless it took action to show that it supported peace in Ukraine.

Earlier, German Chancellor Angela Merkel warned Russia of new sanctions after fresh clashes in eastern Ukraine.

Ukraine's president has said continued violations might end a truce between the government and pro-Russian rebels.

Chancellor Merkel and French President Francois Hollande spoke to Mr Putin and Ukrainian President Petro Poroshenko for an hour on Wednesday, officials said.

They used the telephone conversation to "encourage" both to "work together, especially in order to put in place a mechanism to oversee the truce", a statement from Mr Hollande's office said.

Ukraine's government said the call ended with an agreement to let OSCE observers monitor the country's border with Russia and for both sides to "exchange hostage lists".

On Tuesday, a Ukrainian military helicopter was shot down near Sloviansk with the loss of nine lives, and there were reports of fighting overnight near the Russian border in Luhansk region.

On Wednesday the Ukrainian military accused the rebels of breaking the ceasefire 44 times since it began.

But Alexander Borodai, prime minister of the self-styled Donetsk People's Republic, has said there is effectively no ceasefire because of government attacks.

The truce is part of Ukraine's plan to end two months of conflict with pro-Russian insurgents who control key buildings in towns and cities across the east.

More than 420 people have been killed in the region since mid-April, the UN estimates.

Bakudaily.az