Ukraine's unilateral ceasefire hangs in the balance with clashes spreading across the separatist east and Moscow rankling Kiev by putting its forces on full combat alert.
The resurgence of violence in the 11-week pro-Russian uprising threatens to splinter the ex-Soviet state.
Washington has also slapped sanctions on separatist leaders and warned the Kremlin against actual deployment.
However, Russian President Vladimir Putin on Saturday appeared ready to continue sabre-rattling by ordering units from the Volga to western Siberia to conduct snap military drills.
"There is no ceasefire," a woman named Lila Ivanovna said just four kilometres southwest of the battled-scarred rebel stronghold city of Slavyansk.
"They were shooting last night and I heard mortar and machinegun fire at four this morning. Nothing has changed."
Ukrainian border guards said the militia used sniper fire and grenade launchers to strike a base in the eastern Donetsk four hours after President Petro Poroshenko declared a unilateral halt to hostilities that have claimed more than 375 lives.
It said troops had to return fire when the same rebel unit mounted a second attack near a different Russian border crossing a few minutes later.
A spokesman for Ukraine's "anti-terrorist operation" confirmed the fighting around Slavyansk while the defence ministry said one of its anti-aircraft bases was attacked by "50 men in camouflage".
Ukraine's SBU security service said nine border guards were wounded in violence overnight.
But the separatist leader of the self-proclaimed Donetsk People's Republic told reporters Slavyansk had absorbed a heavy air and artillery assault from Ukrainian troops.
Poroshenko ordered his forces to hold fire for a week on Friday evening as part of a broader peace plan that would eventually give more rights to the eastern industrial region which has strong Russian ties.
But the Western-backed leader also cautioned that "this does not mean that we will not fight back against aggression toward our troops".
"We know how to protect our nation," he told wounded soldiers during a visit on Saturday to a Kiev military hospital.
Russia dismissed Poroshenko's plan as an "ultimatum" that left rebels with a choice between complete surrender or an even more aggressive Ukrainian army push.
"The fact that the so-called counter-terrorist military operation has intensified in parallel with the advancement of a peace plan is a cause for much alarm and concern," Russian Foreign Minister Sergei Lavrov said on a visit on Saturday to Saudi Arabia.
Just two weeks into his term, Poroshenko's attempts to resolve the country's worst post-Soviet crisis have also been complicated by a new deployment of Russian forces along parts of the border where the rebels mount the most frequent attacks.
Putin appeared to be stirring tensions further on Saturday by ordering troops stretching from the Volga region in central Russia to the Ural Mountains and swathes of Siberia to go on "full combat alert" as part of an unannounced readiness check.
The Russian defence ministry said military exercises in the expansive region whose western-most edge lies 400 kilometres east of Ukraine would involve 65,000 soldiers along with 60 helicopters and 180 jets.
But both Kiev and its Western allies are also anxious about the presence of new Russian forces along the border amid charges of growing flows of heavy weapons into parts of the rebel-held industrial east.
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