According to Ukrainian officials, pro-Russian separatists have carried out several attacks on Ukrainian troops despite a unilateral ceasefire declared by Kiev.
They say at least six border guards had been injured in shelling in the east.
The rebels earlier dismissed the week-long truce called by President Petro Poroshenko, the first step of his peace plan announced on Friday.
On Saturday, however, Russian President Vladimir Putin said he backed the plan – if it included talks with the rebels.
Vladimir Putin also said Ukraine must make compromises with the rebels.
He repeated the Kremlin’s earlier criticism of the plan, saying that the temporary ceasefire which accompanied the proposals must not be offered as an ultimatum.
Meanwhile, the US imposed sanctions – including asset freezes – against seven pro-Russian leaders in Ukraine.
Western leaders have threatened additional sanctions against Russia, which they accuse of stoking tension in Ukraine. Moscow denies the claim.
In a separate development, Vladimir Putin ordered forces in Russia’s central military district to be put on full combat alert for a week.
The drill does not affect troops near the border with Ukraine. Vladimir Putin has ordered several such alerts to test combat readiness in recent months.
Overnight, the separatists attacked three Ukrainian border posts in the Donetsk and Luhansk regions, Kiev said.
The rebels shelled the posts with rocket and mortar fire, injuring six border guards.
The gunmen also attacked Ukrainian forces outside the Kramatorsk airfield, in the Donetsk region.
The Ukrainian forces returned fire, and all the attacks were repelled, Kiev said. However, Ukraine’s border guard service said its personnel at the Izvaryne post, Luhansk region, were forced to retreat into Russian territory.
A number of casualties were reported among the separatists.
President Petro Poroshenko on Friday declared the ceasefire to give rebels time to disarm. It became effective from 22:00 local time.
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