Nato's chief has dismissed as "nonsense" a claim by Russia's Vladimir Putin that the Ukrainian army operates as a "foreign legion" for the alliance.
Nato Secretary General Jens Stoltenberg also called on Russia to stop providing military support to pro-Russian rebels.
President Putin said Ukraine's army was operating against Ukrainian national interests by seeking to contain Russia.
Violence in eastern Ukraine is at its worst level for months, with a series of deadly attacks over the past week.
The military said early on Monday that seven Ukrainian soldiers had been killed fighting Russian-backed separatists in 24 hours.
It said the heaviest fighting was continuing around the town of Debaltseve.
On Saturday, a series of rocket attacks left 30 people dead and many more injured in the Azov Sea city of Mariupol.
Ukraine's security service on Monday released a video in which it presented what it described as a captured artillery spotter for the Mariupol attacks.
The footage showed the man - a Ukrainian national identified as Valeriy Kirsanov - admitting he guided the strikes which had been carried out on orders by a Russian regular army officer.
The rebels blame the attacks on Ukrainian troops, calling them a provocation.
Nato and Ukrainian officials were meeting in Brussels to discuss the recent surge in fighting.
"The statement that there is a Nato legion in Ukraine is nonsense," Mr Stoltenberg said. "There is no Nato legion, the foreign forces in Ukraine are Russian."
The Nato chief also urged Russia to stop providing backing to rebels, saying hundreds of pieces of advanced weaponry including tanks, heavy artillery and armoured vehicles had crossed the border.
He was responding to comments that Mr Putin made to students in St Petersburg.
"In effect, it is no longer an army but a foreign legion, in this case Nato's foreign legion, which does not of course pursue the aims of Ukraine's national interests," said the Russian president.
Those aims were "connected with achieving the geopolitical objectives of containing Russia", he said.
Russian Foreign Minister Sergei Lavrov said the Kiev government had tried "to derail the peace process... again and again".
There have been warnings from the West that existing sanctions against Russia could be stepped up in response to the latest violence. EU foreign ministers are due to hold an extraordinary meeting on Ukraine on Thursday.
Mr Putin's spokesman, Dmitry Peskov, accused the West of "economic blackmail".
During ongoing violence around Donetsk, nearly 500 miners were briefly trapped after shelling hit an electricity substation and cut power to their mine.
Emergency workers said all the miners had been rescued.
Rebel leader Alexander Zakharchenko said on Saturday that separatist forces intended to surround the government-held Debaltseve, where the military said the violence had been most intense overnight into Monday.
Mr Zakharchenko also initially said his troops had begun an offensive to retake the strategically important city of Mariupol, but later backtracked and blamed Ukrainian forces for the deaths there.
OSCE observers said analysis of a crater in Mariupol indicated that the rockets had been fired from rebel-held territory. Ukrainian President Petro Poroshenko said intercepted radio and telephone conversations proved separatists were behind the attacks.
More than 5,000 people have been killed in fighting since the rebels seized swathes of Donetsk and Luhansk regions last April, according to UN estimates. More than a million people have been displaced.
A ceasefire agreed as part of a peace deal signed in September has been repeatedly violated by both sides.
Ukraine and the West have accused Russia of arming the rebels and sending its troops into Ukrainian territory.
Russia has denied directly arming the separatists, and blames Ukraine for the surge in fighting.
The OSCE said its permanent council would meet in Vienna on Monday "in light of the rapid deterioration of the situation in eastern Ukraine".
Ukraine: the human cost
- Some 5.2 million people live in conflict-affected areas and 1.4 million are considered "highly vulnerable and in need of assistance"
- More than a million people have fled their homes with 633,523 living as displaced persons within Ukraine and 593,622 living outside Ukraine, mostly in Russia
- More than 5,000 people have been killed in the fighting and more than 10,300 injured
Source: UN report of 9 January for refugee figures; news reports for casualty estimates