Fierce battles on Ukraine's porous eastern border left have 15 seven soldiers and eight border guards dead in the past 24 hours as fears of a possible Russian invasion continued despite NATO urging Moscow to withdraw its troops along the frontier.
The latest deaths mean the number of government forces killed in fighting in the east has passed 400.
Relations between Moscow and the West are at a post-Cold War low, with NATO saying Russia has massed 20,000 troops near the border with Ukraine.
International tensions also rose as Western countries slammed a Russian food embargo imposed as revenge for sanctions slapped on Moscow over its backing for separatist fighters in Ukraine.
The renewed violence came after NATO chief Anders Fogh Rasmussen warned Moscow to "pull back from the brink" and as Western countries warned that Russia could be preparing to send troops across the border in the guise of a humanitarian mission.
Late on Friday, Philip Hammond, the British Foreign Secretary, said that he was deeply concerned by reports of an increased flow of heavy weapons crossing into Ukraine from Russia and by reports of Russian armed forces exercising for a "humanitarian intervention" in a third country.
He was responding to Ukrainian military reports on Friday that Russian military vehicles have crossed the border from Russia into Ukraine.
"We have consistently called on Russia to stop the flow of weapons across the border and made clear that the international community will increase the cost to Russia if it fails to take steps towards de-escalation," Hammond said.
On a possible Russian intervention in Ukraine, he said: "The conditions for such an intervention in eastern Ukraine manifestly do not exist. In these circumstances, such an intervention would be unjustified and illegal."
Polish Prime Minister Donald Tusk said this week he suspected the threat of a direct intervention by Russia's military in Ukraine had increased recently.
Western countries accuse Russia of stoking the conflict between the separatists and Ukrainian government forces in eastern Ukraine and have imposed several rounds of sanctions on Moscow.
Moscow denies arming the rebels and has retaliated by restricting food imports from many Western countries.
Russian President Vladimir Putin met with his national security council on Friday to discuss the situation in eastern Ukraine, especially the "massive humanitarian catastrophe" in the region.
The US government said it would see any bid by Russia to deliver humanitarian aid into Ukraine as an invasion after Russia offered to send a convoy of aid across the border for displaced civilians.
"Given that Ukraine has allowed international humanitarian groups to deliver aid within its territory, there is no logical reason why Russia should seek to deliver it," US Ambassador to the UN Samantha Power told a UN Security Council meeting on Ukraine on Friday.
"Therefore, any further unilateral intervention by Russia into Ukrainian territory, including one under the guise of providing humanitarian aid, would be completely unacceptable and deeply alarming.
"And it would be viewed as an invasion of Ukraine," Power told the 15-member body.
Russia's Defence Ministry said on Friday it had finished military exercises in southern Russia which various Western countries criticised as a "provocative" step amid the Ukraine crisis.
"Aircraft taking part in exercises have been redeployed from temporary to their permanent air bases, anti-aircraft missile units ... have started to load their equipment to depart to their permanent positions," Russia’s Interfax news agency quoted the ministry's news service as saying.
The ministry added that the drills in the southern Astrakhan region, located 1,000km from the border with Ukraine, had shown a "high level of cohesiveness" among troops.