Fierce battles on Ukraine's porous eastern border left 15 government troops dead as fears of a possible Russian invasion swirled Saturday with NATO urging Moscow to withdraw its troops along the frontier.
International tensions also rose as Western countries slammed a Russian food embargo imposed as revenge for sanctions slapped on Moscow over its backing for insurgents 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.
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.
Council Secretary Nikolai Patrushev vowed that Moscow was trying its best "to de-escalate tensions", but Kiev said its positions continued to come under fire from Russian territory.
Ukraine's military said seven soldiers and eight border guards were killed over the past 24 hours as a bloody three-day battle with pro-Russian rebels forced several government units to retreat from the border in the southeast of the war-torn Lugansk region.
An AFP journalist meanwhile heard renewed sporadic shelling in the main insurgent bastion of Donetsk.
The centre of the city has become a new battleground in the fighting, coming under sustained shelling for the first time on Thursday, with mortar fire killing at least three civilians and hitting a hospital.
But the rebels, believed by the West to be backed by Russia, have not stepped back in the face of a fierce government assault.
The new prime minister of the self-proclaimed Donetsk People's Republic, Alexander Zakharchenko, admitted that the situation was "difficult and tense" but said "the troops' morale is strong".
More than 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 in eastern Ukraine.
Local authorities have warned of an impending humanitarian disaster, as some areas have been left without power or water for days, and fuel and food were running short.
NATO slams 'Russian aggression
Aside from the 15 government troops killed, 79 soldiers were wounded in the east in the past 24 hours, Ukraine's military said on Friday. The rebels shot down a Ukrainian fighter jet the day before some 40 kilometres (25 miles) east of Donetsk and said they had captured the two pilots. They also opened fire on a medical evacuation helicopter, injuring the three crew on board and forcing it to make an emergency landing, Kiev said.
Ukraine's military said on Friday that its forces had reclaimed two more villages from the insurgents. They have been seeking to cut off Donetsk from the Russian border, where Moscow has amassed some 20,000 troops, according to NATO.
Fears have mounted that Russia could be preparing to send the troops into Ukraine under the pretext of a humanitarian mission.
NATO chief Rasmussen appealed to Moscow on Thursday to "not use peacekeeping as an excuse for warmaking" and vowed his support for Ukraine against what he called "Russian aggression".
The United States warned Russia Friday against using the humanitarian crisis as a pretext for an invasion, saying such a move would be "completely unacceptable".
US Ambassador Samantha Power also told the UN Security Council that international aid agencies were on the ground and ready to help civilians fleeing fighting between pro-Moscow rebels and Ukrainian forces.
"Any further unilateral intervention by Russia in Ukrainian territory, including one under the guise of providing humanitarian aid, would be completely unacceptable and deeply alarming and would be viewed as an invasion of Ukraine," Washington's envoy said.
Canada meanwhile said it was sending protective vests, first aid kits and tents to help Kiev "secure and protect its eastern border against Russian aggression".
And UN Secretary-General Ban Ki-moon said in a statement the world body was ready to boost its support to Ukrainian relief efforts.
"If there is further deterioration of the humanitarian situation, or if nationally led response efforts are not sufficient to meet humanitarian needs, the UN is ready to consider additional measures of support," he said.
Away from the fighting, Moscow also drew condemnation for its decision to block most food imports from countries that have placed sanctions on Russia over the crisis.
France has asked the European Commission to take "appropriate" measures to deal with the fallout from the ban. The request came as President Francois Hollande met with French food producers.
The French presidency said that an approach to the World Trade Organization (WTO) should also be considered.
Brussels said it could consider "action" against the Russian embargo while Australian Prime Minister Tony Abbott warned that Canberra was "working towards stronger sanctions".
In a separate set of punitive measures, Ukrainian Prime Minister Arseniy Yatsenyuk announced a new sanctions list targeting 65 mainly Russian companies and 172 individuals accused of backing the separatists. The list must first be approved by parliament.