MOSCOW (REUTERS) - Russian Foreign Minister Sergei Lavrov told US Secretary of State John Kerry on Friday it was essential that both Ukrainian forces and pro-Russian separatists fully observed a ceasefire in eastern Ukraine clinched nearly a month ago.

The ceasefire, signed by the separatists and Kiev's representatives in Minsk on Sept 5, has increasingly frayed in recent days, leading to the deaths of civilians, soldiers and a Red Cross aid worker in the rebel stronghold of Donetsk.

Both sides regularly accuse each other of violating the ceasefire.

"(Lavrov) underscored the necessity of a strict observance by the fighting parties of the Minsk accords on a ceasefire," the Russian Foreign Ministry said in a statement after his telephone conversation with Mr Kerry.

Mr Lavrov also stressed the need to withdraw heavy weapons from the area and the role of Europe's OSCE security watchdog in monitoring the situation on the front line, it said.

Under the peace plan, the two sides are supposed to move heavy weapons away from the front line and to create a 30km buffer zone, with monitors from the Organisation for Security and Cooperation in Europe acting as monitors.

Earlier on Friday, Ukraine accused Russian forces of helping the separatists step up pressure on its own troops holding the main airport of Donetsk.

Russia denies accusations by Ukraine and its Western backers of sending troops across the border in support of the rebels or arming them, despite what Kiev says is incontrovertible proof.

Moscow said mortar fire that killed a Swiss worker from the International Committee of the Red Cross in Donetsk late on Thursday was fired from territory controlled by Ukrainian government forces.

The conflict has plunged ties between Russia and the West, which backs Kiev, to their lowest level since the end of the Cold War and claimed at least 3,000 lives, including 298 killed when a Malaysian airliner was downed over rebel-held territory.

A Ukrainian military spokesman said on Friday at least 30 government soldiers have been killed since the start of the ceasefire and more than 10 people, including rebels and the Red Cross aid worker, have been killed in Donetsk from shelling.