Ukraine's Poroshenko says rebels ousted from Maryinka
Ukraine's president says his forces have ousted pro-Russia rebels from the eastern town of Maryinka and captured 12 "saboteurs", including one Russian.
Petro Poroshenko's claim has not been independently confirmed.
Heavy fighting erupted on Wednesday in Maryinka and Krasnohorivka, west of rebel-held Donetsk.
The opposing sides have accused each other of shattering February's Minsk ceasefire, requiring them to withdraw heavy weapons from the frontline.
The Donetsk rebel leader spoke of huge losses in the Ukrainian army.
Alexander Zakharchenko, leader of the self-styled "Donetsk People's Republic", said the Maryinka fighting had left about 400 Ukrainian troops dead and up to 1,000 wounded.
"This was a counter-attack. If we had attacked, we would have captured Maryinka," he said on Friday.
Quoted by Russia's RIA Novosti news agency, he said 15 rebel fighters had been killed and 30 wounded.
Ukrainian officials said three government soldiers and about 80 rebels were killed in the intense fighting.
Tanks and other heavy weapons were used in the clashes, despite the terms of the Minsk accord agreed in February demanding that both sides pull heavy weapons far back from the frontline.
President Poroshenko told a news conference on Friday that Russia had massed troops on the border and in rebel-held areas "in unprecedented numbers" - but Russia again denied that its military was involved in Ukraine.
Mr Poroshenko said Ukraine has deployed 50,000 troops in the conflict zone to meet the threat.
He also said he would not allow any Russian forces to transit Ukraine in order to reach the Russian army in Trans-Dniester. Pro-Russian separatists run the breakaway region on Ukraine's western border, which remains legally part of Moldova.
The White House said US President Barack Obama spoke with his Ukrainian counterpart on Friday about the violence in the east of the country.
It said both leaders expressed their "deep concern" about fighting near Donetsk and urged rebels to respect the Minsk ceasefire.
Their concern was echoed by Alexander Hug, deputy chief of the OSCE monitoring mission in Ukraine, who told the UN Security Council that there had been "a significant deterioration of the security situation".
"The violence witnessed in and around the town of Maryinka close to the [frontline] constitutes a new worrying development in the conflict in eastern Ukraine," he said.
"Our assessment from the ground is that tension levels have increased and that ceasefire violations are becoming more frequent as well as more severe."
More than 6,400 people have been killed in eastern Ukraine since the conflict began in April 2014, when rebels seized large parts of two eastern regions, following Russia's annexation of the Crimea peninsula.
High stakes in the east - by Tom Burridge, BBC News, Donetsk
If there is a spike in fighting, like the battle in the town of Maryinka on Wednesday, then both sides know they cannot be seen as the aggressor, because they lose credibility and damage the negotiating position of their allies in either Moscow, or in European capitals.
Neither side wants to be seen as responsible for breaking the highly publicised, but so far unsuccessful, Minsk peace agreement.
And bargaining power for either side will become ever more crucial because in three weeks the European Union will decide whether to renew sanctions against Russia.