Fierce battles on Ukraine's porous eastern border left 15 government servicemen dead as fears of a possible invasion swirled after NATO urged Moscow to withdraw its troops along the frontier.
International tensions also rose as Western countries hit out at a Russian food embargo imposed as revenge for sanctions slapped on Moscow over its backing for insurgents in Ukraine.
Kiev said seven soldiers and eight border guards were killed over the past 24 hours as a bloody three-day battle with pro-Russian rebels eventually forced several government units to retreat from the border in the southeast of the war-torn Lugansk region.
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.
Heavy shelling again rocked the main insurgent bastion of Donetsk today, an AFP journalist reported.
The centre of the one-million strong city appears to have become a new battleground in the fighting, coming under sustained shelling for the first time yesterday, with mortar fire hitting a hospital and residences near a key rebel base.
The rebels, believed by the West to be backed by Russia, have not stepped back in the face of a fierce government assault.
Yesterday they downed a Ukrainian fighter jet and fired on a medical evacuation helicopter.
The new prime minister of the self-proclaimed Donetsk People's Republic, Alexander Zakharchenko, who took over yesterday, admitted that the situation was "difficult and tense" but said "the troops' morale is strong."
Over 1,300 people have been killed and 285,000 forced to flee their homes over four months of what the Red Cross has designated a civil war.
Local authorities have warned of an impending humanitarian catastrophe, as some areas in the east have been left without power or water for days, and fuel and food were running short.
Ukraine's military said that in addition to those killed, 79 soldiers were injured in the east in the past 24 hours and that its forces had reclaimed two more villages from the insurgents.
In a statement the army said the rebels had opened fire on a medical evacuation helicopter, injuring the three pilots and forcing it to make an emergency landing.
This came after rebels shot down a Ukrainian fighter jet yesterday, some 40 kilometres east of Donetsk, and said they had captured the two pilots.
Ukrainian forces have been seeking to retake Donetsk and other rebel-held cities and cut them off from the Russian border, where Moscow has amassed some 20,000 troops according to NATO.
