European Council president Antonio Costa and Kaja Kallas, the European Union's foreign policy chief, have arrived in the Ukrainian capital Kyiv, using the first day in their new roles to send a message of support for Ukraine in its war with Russia.
Their visit comes as Ukraine struggles to fend off a grinding Russian offensive and faces the uncertainty of US policy toward it when Donald Trump takes office next month.
"From day one of the war, the EU has stood by the side of Ukraine," Costa posted on X alongside an image of himself, Kallas and EU enlargement chief Marta Kos arriving via train.
"From day one of our mandate, we are reaffirming our unwavering support to the Ukrainian people."
Both Kallas and Costa have been strong supporters of Ukraine since Russia's February 2022 invasion.
However, neither can make specific pledges of further aid, requiring the support of the EU's member countries.
The EU says its institutions and member countries have made available $US133 billion ($A204 billion) in Ukraine aid since the start of the war but future support remains uncertain especially if Trump reduces US support.
Trump has criticised the scale of military aid for Ukraine and has said he will seek a swift end to the war but without specifying exactly how.
President Volodymyr Zelenskiy said on Sunday there was still time for the United States to convince "sceptics" in Europe that Ukraine should be invited to join the NATO military alliance.
He told a news conference in Kyiv that any invitation to join the alliance must apply to all of Ukraine's territory but accepted that NATO's defence umbrella could not apply to the occupied parts of Ukraine with the war ongoing.
On the battlefield, Russian troops are capturing village after village in a drive to eventually seize the industrial Donbas region while Russian air strikes are targeting Ukraine's hobbled energy grid as winter sets in.
"In my first visit since taking up office, my message is clear: the European Union wants Ukraine to win this war," Kallas wrote on X.
"We will do whatever it takes for that."
As prime minister of Estonia, which borders Russia, Kallas emerged as one of the most vociferous critics of Russia.
Russia this year put her on a wanted list for destroying Soviet-era monuments.
Costa, a former prime minister of Portugal, is tasked with co-ordinating the work of the European Union's national leaders and chairing their summits as president of the European Council.
At a ceremony in Brussels on Friday, he said everyone was yearning for peace after more than 1000 days of the Ukraine-Russia war, "especially the embattled and heroic Ukrainian people".
"Peace cannot mean capitulation. Peace must not reward the aggressor," he added.
The US is not considering returning to Ukraine the nuclear weapons it gave up after the Soviet Union collapsed, White House national security adviser Jake Sullivan said on Sunday.
Sullivan made his remarks when questioned about a New York Times article last month that said some unidentified officials had suggested US President Joe Biden could give Ukraine the arms before he leaves office.
"That is not under consideration, no. What we are doing is surging various conventional capacities to Ukraine so that they can effectively defend themselves and take the fight to the Russians, not (giving them) nuclear capability," he told US broadcaster ABC.
Last week, the Kremlin said the idea was "absolute insanity" and that preventing such a scenario was one of the reasons why Russia sent troops into Ukraine.
