Kiev and pro-Russian rebels are supposed to pull back their forces and weaponry to create a 30-kilometre (20-mile) wide buffer zone in line with a reinforced truce deal agreed in Minsk yesterday.
But heavy gunfire erupted around the eastern insurgent stronghold of Donetsk today, just hours after NATO's top military commander had said there was a ceasefire "in name only" on the ground.
National Security and Defence Council spokesman Andriy Lysenko said the first point agreed in the Belarussian capital referring to a mutual ceasefire had to be respected before the rest of the plan could be implemented.
"If (Ukrainian forces) are withdrawn, it will be done simultaneously with the Russian troop withdrawal."
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The nine-point Minsk plan is meant to reinforce a truce forged on September 5 in a bid to stem fighting that has claimed nearly 3,000 lives since April and threatened Ukraine's very survival.
A rebel who identified himself as a member of Donetsk People's Republic's military intelligence said Ukrainian forces were continuing to shoot on insurgent positions around the airport although the attacks had dropped off in intensity and firepower.
NATO's top commander General Philip Breedlove said yesterday the continued clashes had shown it was a ceasefire "in name only" and accused Russia of keeping soldiers on Ukrainian soil to bolster the insurgents.
The truce was "still there in name, but what is happening on the ground is quite a different story," he said on the sidelines of a NATO meeting symbolically convened in the ex-Soviet satellite state of Lithuania.
But he added: "It is our sincere hope and desire that... the two combatants can come to agreement to again get to a ceasefire situation."
There is no information on casualties from the rebel side.