Ukrainian and Russian negotiators met in Abu Dhabi on Friday to tackle the vital issue of territory, with no sign of a compromise, as Russian airstrikes plunged Ukraine into its worst energy crisis of the nearly four-year war.

Kyiv is under mounting US pressure to reach a peace deal in the war triggered by Russia's full-scale invasion in February 2022, with Moscow demanding Kyiv cede its entire eastern industrial area of Donbas before it stops fighting.

Ukrainian President Volodymyr Zelenskiy said the territorial dispute was a central issue for the tripartite talks, including Russian, Ukrainian and US officials, which were scheduled to conclude on Saturday.

"The most important thing is that Russia should be ready to end this war, which it started," Zelenskiy said in a statement on the Telegram app, adding he was in regular contact with the Ukrainian negotiators, but it was too early to draw conclusions from Friday's talks.

"We'll see how the conversation goes tomorrow and what the outcome will be."

Rustem Umerov, the secretary of Ukraine's National Security and Defence Council and the head of its delegation, said in a statement the talks had discussed parameters for ending the war and the "further logic of the negotiation process."

The negotiations come a day after Zelenskiy met with US President Donald Trump at the World Economic Forum in Davos, Switzerland.

Zelenskiy said on Friday that a deal on US security guarantees for Ukraine was ready, and that he was only waiting on Trump for a specific date and place to sign it.

Ukraine has sought robust security guarantees from Western allies in the event of a peace deal to prevent Russia, which has shown little interest in ending the war, from invading again.

Russia steps up attacks on power infrastructure

The tripartite talks, brokered by the US, are unfolding against a backdrop of intensified Russian strikes on Ukraine's energy system that have cut power and heating to major cities such as Kyiv, as temperatures dip well below freezing.

The head of Ukraine's top private power producer, Maxim Timchenko, told Reuters on Friday the situation was nearing a "humanitarian catastrophe" and that Ukraine needs a ceasefire that halts attacks on energy infrastructure.

Kyiv's energy minister said on Thursday that Ukraine's power grid had endured its most difficult day since a widespread blackout in November 2022, when Russia began bombing energy infrastructure.

Russia says it wants a diplomatic solution but will keep working to achieve its goals by military means as long as a negotiated solution remains elusive.

Russian President Vladimir Putin's demand that Ukraine surrender the 20% it still holds of the Donetsk region of the Donbas - about 5,000 sq km (1,900 sq miles) - has proven a major stumbling block to a breakthrough deal.

Zelenskiy refuses to give up land that Russia has not been able to capture in four years of grinding, attritional warfare. Polls show little appetite among Ukrainians for territorial concessions.

Kremlin spokesman Dmitry Peskov said on Friday that Russia's insistence on Ukraine yielding all of Donbas was "a very important condition."

A source close to the Kremlin told Reuters that Moscow considers an "Anchorage formula" - which Russia says was agreed between Trump and Putin at a summit in Alaska last August - would hand Russia control of all of Donbas and freeze the front lines elsewhere in Ukraine's east and south.

Donetsk is one of four Ukrainian regions Moscow said in 2022 it was annexing after referendums rejected by Kyiv and Western nations as bogus. Most countries recognise Donetsk as part of Ukraine.

Moscow wants use of frozen assets

Russia has also floated the idea of using the bulk of nearly $5 billion of Russian assets frozen in the United States to fund a recovery of Russian-occupied territory inside Ukraine. Ukraine, backed by European allies, demands that Russia pay it reparations.

Asked about Russia's idea, Zelenskiy dismissed it as "nonsense."

Zelenskiy said on Thursday in Davos that the Abu Dhabi talks would be the first trilateral meetings involving Ukrainian and Russian envoys and US mediators since the war began.

Last year, Russian and Ukrainian delegations had their first face-to-face meeting since 2022 when they met in Istanbul. A top Ukrainian military intelligence officer also had talks with US and Russian delegations in Abu Dhabi in November.