ANDRIYIVKA, Ukraine (AFP) - Ukraine's unilateral ceasefire hung in the balance on Saturday as clashes engulfed the separatist east and Russian President Vladimir Putin questioned the new Western-backed leader's claimed commitment to peace.

The resurgence of violence in the 11-week pro-Russian uprising threatening to splinter the ex-Soviet state came as Washington accused the Kremlin of covertly arming the rebels and sternly warned Putin against sending troops into Ukraine.

But the Russian strongman appeared ready to continue sabre-rattling in the worst East-West standoff since the Cold War 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 south-west 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 region 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 battles around Slavyansk while the defence ministry said one of its anti-aircraft bases was assaulted 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 that 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 eastern industrial regions where pro-Russian sentiments run high.

The 48-year-old has followed through on his May 25 election promise to ignore Kremlin pressure and bring Ukraine into the Western fold by signing an historic EU trade agreement in Brussels on Friday.

But Poroshenko's peace initiative includes a major caveat that allows the military to retaliate with equal force against any attack.
"We know how to protect our nation," he told wounded soldiers during a visit to a Kiev military hospital.

Putin issued a carefully-worded statement on Saturday saying he "supports... Poroshenko's decision to halt fire in the southeast of Ukraine." But the Kremlin chief also stressed that any peace initiative "not aimed at starting the negotiating process will not be viable or realistic".

Putin also called on "the conflicting parties to halt all military activities and sit down at the negotiating table" - a comment that implied a degree of criticism for the rebels' continued attacks.

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 a surprise readiness check.

The Russian defence ministry said military exercises in the expansive region whose western-most edge lies 400 kilometres (250 miles) 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 crossing into rebel-held parts of the industrial east.

Foreign ministry spokesman Yevgen Perebiynis told AFP that Putin's order for snap drills in central Russian "cannot but also raise concern if it is in any way linked to earlier deployments near the border."

Ukrainian officials have told EU and G7 teams in Kiev that they have evidence of 10 additional tanks and sealed trucks coming over the border close to the eastern city of Lugansk since Thursday.

A Russian defence ministry source told the RBK news agency this week that troops were prepared to enter Ukraine's insurgent regions in order to "put up barriers between the civilian population and the Ukrainian army".

"We will not accept the use, under any pretext, of any Russian military forces in eastern Ukraine," said White House spokesman Josh Earnest.

US State Department spokesman Jen Psaki noted that most of the equipment being gathered in south-west Russia was no longer used by its military.

"We believe that Russia may soon provide this equipment to separatist fighters," Ms Psaki said.