Ukraine is under mounting US pressure to reach a peace deal in the war triggered by Russia's full-scale invasion in February 2022, with the Kremlin demanding Ukraine cede its entire eastern industrial area of Donbas before it stops fighting.
Ukrainian President Volodymyr Zelenskiy said the territorial dispute would be a top priority of the talks in the United Arab Emirates.
"The question of Donbas is key. It will be discussed how the three sides ... see this in Abu Dhabi today and tomorrow," he told reporters in a WhatsApp chat a day after talks with US President Donald Trump at the World Economic Forum in Davos.
The negotiations in the Gulf are expected to continue on Saturday morning, Zelenskiy's aide said.
The talks unfold against a backdrop of intensified Russian strikes on Ukraine's energy system that have cut power and heating to major cities like Kyiv as temperatures hover well below freezing.
Russian President Vladimir Putin's demand that Ukraine surrender the 20 per cent it still holds of the Donetsk region of the Donbas - about 5,000 sq km - 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 indicate little appetite among Ukrainians for territorial concessions.
Kremlin spokesman Dmitry Peskov said on Friday that Russia's insistence on Ukraine yielding the Donbas was "a very important condition".
A source close to the Kremlin told Reuters that Russia considers a so-called "Anchorage formula," which officials said was agreed between Trump and Putin at a summit last August, to mean Russia controlling all of Donbas and freezing the current front lines elsewhere in Ukraine's east and south.
Donetsk is one of four Ukrainian regions Russia said in 2022 it was annexing after referendums rejected by Ukraine and its allies as bogus.
Most countries recognise Donetsk as part of Ukraine.
As Friday's talks proceeded, the head of Ukraine's top private power producer, Maxim Timchenko, told Reuters that Ukraine needs a ceasefire that halts attacks on energy, saying the situation was nearing a "humanitarian catastrophe".
Ukraine'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 first began bombing energy infrastructure.
Zelenskiy said on Thursday in Davos that the UAE 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.
Ukraine has sought robust security guarantees from its allies in the event of a peace deal to prevent Russia from invading again.
Zelenskiy also told reporters 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.
For its part, Russia has floated the idea of using the bulk of nearly $US5 billion ($A7.3 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".
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.
