Ukraine will continue fighting until it has recovered all of its territory, its president Volodymyr Zelensky has vowed.
Mr Zelensky said he would only be willing to speak directly to Vladimir Putin about a diplomatic end to the conflict and not through intermediaries.
Speaking at the World Economic Forum in Davos, Mr Zelensky said if the Russian leader “understands reality" there was the possibility of finding a diplomatic solution, but Moscow should withdraw its troops out of Ukraine’s pre February 24 territory.
“That might be a first step towards talks," he said, adding that Russia has been playing for time in its talks with Ukraine.
It comes as Russian troops seek to encircle Ukrainian forces in eastern cities in the industrial Donbas region.
Russia is attempting to seize the separatist-claimed Donbas' two provinces, Donetsk and Luhansk, and trap Ukrainian forces in a pocket on the main eastern front.
"The situation in Donbas is extremely difficult. All the remaining strength of the Russian army is now concentrated on this region," Mr Zelenskiy said in a late Tuesday address.
“The occupiers want to destroy everything there."
Musa Magomedov, a Ukrainian member of parliament, said Russian forces were shelling Avdiivka, a town just inside the Ukrainian-controlled part of Donetsk, “in totally inhuman fashion”.
“They are killing civilians and destroying the city we built up with such love," Mr Magomedov said.
Ukraine's military general staff said Russia had intensified the use of aircraft to support ground forces because it lacked supplies of high-precision missiles.
Russian forces took control of three towns in the Donetsk region including Svitlodarsk, regional governor Pavlo Kyrylenko said.
But elsewhere in Ukraine, authorities in its second-largest city, Kharkiv, re-opened the underground metro, where thousands of civilians had sheltered for months under relentless bombardment.
Residents formed long lines to receive rations of flour, pasta, sugar and other staples this week.
Moscow's forces withdrew from around Kharkiv earlier this month, but it still continues to shell the area from afar.