US Army Secretary Dan Driscoll has held talks with Russian officials in Abu Dhabi as part of an intense new push by President Donald Trump's administration to end the war in Ukraine.
US and Ukrainian officials are trying to narrow the gaps between them over a peace plan, with core issues still unresolved and Ukraine wary of being strong-armed into accepting a deal largely on the Kremlin's terms.
"Late Monday and throughout Tuesday, Secretary Driscoll and team have been in discussions with the Russian delegation to achieve a lasting peace in Ukraine," said Driscoll's spokesperson, US Army Lieutenant Colonel Jeff Tolbert.
"The talks are going well and we remain optimistic. Secretary Driscoll is closely synchronised with the White House ... as these talks progress."
The exact nature of the discussions was not immediately clear, and it was not known who was in the Russian delegation.
A US official said Driscoll, who has emerged as a point man for US diplomatic efforts on Ukraine, was also expected to meet Ukrainian officials while in Abu Dhabi.
Underlining the high stakes for Ukraine, its capital Kyiv was hit by a barrage of missiles and hundreds of drones overnight in a Russian strike that killed at least six people and disrupted power and heating systems.
Residents were sheltering underground wearing winter jackets, some in tents.
US policy towards the war has zigzagged in recent months.
A hastily arranged summit between Trump and Russian President Vladimir Putin in Alaska in August raised worries in Kyiv and European capitals that the Trump administration might accept many Russian demands, though ultimately resulted in more US pressure on Russia.
The latest US peace proposal, a 28-point plan that emerged last week, caught many in the US government, Kyiv and Europe off-guard and prompted fresh concerns that the Trump administration might be willing to push Ukraine to sign a peace deal heavily tilted toward Moscow.
The plan would require Kyiv to cede more territory, accept curbs on its military and bar it from ever joining NATO - conditions Kyiv has long rejected as tantamount to surrender.
The sudden push raises the pressure on Ukraine and President Volodymyr Zelenskiy, who is now at his most vulnerable since the start of the war after a corruption scandal saw two of his ministers dismissed, and as Russia makes battlefield gains.
Zelenskiy could struggle to get Ukrainians to swallow a deal viewed as selling out their interests.
He said on Monday the latest peace plan incorporated "correct" points after talks over the weekend in Geneva.
"The sensitive issues, the most delicate points, I will discuss with President Trump," Zelenskiy said in his nightly video address.
Zelenskiy, who could visit the US in the next few days, said the process of producing a final document would be difficult. Russia's unrelenting attacks on Ukraine have left many sceptical about how peace can be achieved soon.
Russian Foreign Minister Sergei Lavrov said an amended peace plan must reflect the "spirit and letter" of an understanding reached between Putin and Trump at their Alaska summit.
A group of nations supporting Ukraine, which is known as the coalition of the willing and includes Britain and France, was set to hold a virtual meeting on Tuesday.
"It's an initiative that goes in the right direction: peace. However, there are aspects of that plan that deserve to be discussed, negotiated, improved," French President Emmanuel Macron told RTL radio regarding the US-proposed plan.
"We want peace, but we don't want a peace that would be a capitulation."
