Russia-backed rebels in eastern Ukraine say they have withdrawn all heavy weaponry from the front lines in the districts they control, thereby fulfilling a key condition of a peace deal hammered out in Minsk last month.

Eduard Basurin, a spokesman for the self-declared Donetsk People's Republic said that as the last step separatist militias withdrew six "Grad" howitzers.

"The withdrawal was carried out under control of the Organisation for Security and Co-operation in Europe (OSCE)," Basurin said according to Russia's Interfax news agency.

The separatists in the neighbouring "Luhansk People's Republic" also said that they finished withdrawal, Interfax reported.

The OSCE, which monitors the withdrawal on both sides, did not confirm the move on Sunday.

Alexander Hug, deputy head of the 450-strong OSCE mission in Ukraine, reiterated that while both sides were moving weapons, it was unclear if they were withdrawing.

"We can say that the weapons are moving behind the lines - but it is too early to say that they will stay there," he told dpa in an interview released Saturday.

Hug expressed "cautious optimism" about the ceasefire.

"We are currently seeing that the first step, the ceasefire, is largely holding along much of the 500-kilometre front line," said Hug.

The government in Kiev said that the ceasefire was largely holding in the restive Donbass region, and that no shots had been fired overnight to Sunday.

It noted that pro-Russian rebels had fired shots at Ukrainian military positions on Saturday, a claim which the rebels dismissed as a response to "provocation".

The withdrawal of heavy weapons, the creation of a demilitarised zone and starting a dialogue between Kiev and rebel groups are part of a deal struck by the leaders of France, Germany, Russia and Ukraine in Minsk last month.

Ukrainian media reported that a government soldier and a photographer were killed in mortar strikes on Saturday, with the warring parties blaming each other for the deaths.