Ukraine's military waged new battles Saturday with pro-Moscow rebels who rejected Kiev's unilateral cease-fire, while the government raised alarm over the Kremlin's decision to put troops across Russia on combat alert.
The resurgence of violence in the 11-week insurgency threatening to splinter the ex-Soviet nation came as Washington slapped sanctions on top separatist leaders and warned the Kremlin against sending forces into Ukraine.
But Russian President Vladimir Putin appeared ready to continue sabre-rattling in the worst East-West standoff since the Cold War era by ordering units from the Volga to western Siberia to conduct snap military drills.
Ukrainian border guards said the militia used sniper fire and grenade launchers to strike one of their bases in the eastern Donetsk region four hours after President Petro Poroshenko declared a unilateral halt to hostilities that have claimed more than 375 lives.
"Three soldiers were injured, one seriously, and equipment damaged," the border guard service said in a statement.
It added that Ukrainian troops had to return fire when the same militia 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 defense ministry said one of its anti-aircraft bases was attacked by "50 men in camouflage".
But the separatist leader of the self-proclaimed Donetsk People's Republic told Russia's RIA Novosti state news agency that Slavyansk had absorbed a heavy air and artillery assault from Ukrainian troops.
Poroshenko declared the week-long unilateral cease-fire Friday evening while stressing that it "does not mean that we will not fight back against aggression toward our troops."
Russian Foreign Minister Sergei Lavrov said on a visit to Saudi Arabia on Saturday that he was concerned that Kiev's military campaign had only "intensified".
Poroshenko's attempts to resolve the country's worst post-Soviet crisis two weeks into his term have also been complicated by a new deployment of Russian forces along parts of the border where the rebels mount 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 parts of Siberia to go on "full combat alert" as part of an unannounced readiness check.
The Russian defense ministry said military exercises in the expansive region whose western-most edge lies 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.
"Such military activities ... do not help normalize the situation in Ukraine and discourage out leaders from implementing their peace initiative," the Ukraine foreign ministry said in a statement.
Foreign ministry spokesman Yevgen Perebiynis told AFP that Putin's order for snap drill in central Russian "cannot but also raise concern if it is in any way linked to earlier deployments near the border."
A Russian defense ministry source told 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."
The United States warned Russia to keep its troop out of Ukraine.
"We are monitoring the situation carefully. We will not accept the use, under any pretext, of any Russian military forces in eastern Ukraine," said White House spokesman Josh Earnest.
State Department spokeswoman Jen Psaki noted that most of the equipment being gathered in southwest Russia was no longer used by its military.
"We believe that Russia may soon provide this equipment to separatist fighters," Psaki said.
The U.S. Treasury Department also blacklisted seven Ukraine separatists .accused of being a threat to the country's peace and sovereignty.
But the measure was largely symbolic because none are known to have U.S. assets that would be frozen under Treasury Department rules.