Ukraine has launched delicate dual-track diplomacy with Russia aimed at averting a debilitating gas cut and ending a bloody separatist insurgency by the end of the week.
The meetings in Brussels and Kiev throw down an immediate challenge to new President Petro Poroshenko's European commitment and vow to preserve the territorial integrity of the splintered ex-Soviet state.
The 48-year-old confectionery tycoon and political veteran promised late on Sunday to end fighting "this week" in Ukraine's economically vital eastern rust belt that has claimed more than 200 lives.
And he affirmed after being sworn in as Ukraine's fifth president on Saturday that Kiev would sign a historic pact with the European Union that would finally wrest it out of Russia's orbit as soon as the end of the month.
But the eight-week insurgency that Kiev and the West accuse Russia of orchestrating raged unabated over the weekend.
Ukrainian military sources said that militants had staged a wave of failed attacks on the international airport in the Russian border city of Lugansk after briefly seizing its counterpart in neighbouring Donetsk late last month.
Intense artillery fire and air bombardments also continued in the rebel Donetsk region stronghold of Slavyansk - an industrial city of 120,000 where many have been sheltering in basements for weeks.
The Ukrainian army also said pro-Russian gunmen had taken several of its soldiers prisoner overnight.
"Some were out in the field, but others were abducted," military spokesman Vladyslav Seleznyov wrote in a Facebook post.
"We are still learning the details of everything that happened."
The EU-mediated gas talks in Brussels come on the eve of a Russian deadline for Ukraine to cover a debt of nearly $US4.4 billion ($A4.76 billion) or have its shipments end on Wednesday.
About 15 per cent of Europe's gas from Russia transits through Ukraine - dependence that EU nations have been trying to limit following similar disruptions in 2006 and 2009.
But analysts said the fuel freeze would also deal a bruising blow to a Ukrainian economy that the IMF already expects to contract by five per cent this year.
Ukraine has refused to pay the bills in protest at Russia's decision to nearly double its neighbour's rates in the wake of the February ouster of Kiev's Kremlin-backed president.
Sources said the pressure on all sides to agree greatly boosted the chances of a compromise being reached on Monday.
"There is a strong likelihood that this really will be the final meeting at which we expect to agree on a schedule of payments for the already delivered gas," a Russian source close to the negotiations told Moscow's Vedomosti business daily.
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