The EU's new top diplomat Kaja Kallas and head of the European Council Antonio Costa are in Kyiv in a symbolic show of support for Ukraine on their first day in office.
"We came to give a clear message that we stand with Ukraine, and we continue to give our full support," Mr Costa said.
The European Union's new leadership team is keen to demonstrate it remains firm on backing Kyiv at a perilous moment for Ukraine nearly three years into its fight against Russia's all-out invasion.
Questions are swirling around the future of US support once Donald Trump assumes office in January and there are fears he could force Ukraine to make painful concessions in pursuit of a quick peace deal.
Meanwhile, tensions have escalated as Russian President Vladimir Putin has threatened to strike government buildings in Kyiv with his new Oreshnik missile after firing it at Ukraine for the first time last month.
He said the move is a response to Ukraine getting the green light to strike inside Russia with US and British missiles, and he has threatened to hit back against the countries supplying the weaponry.
It is a privilege to be in Kyiv alongside @eucopresident Costa and Commissioner Kos.
— Kaja Kallas (@kajakallas) December 1, 2024
In my first visit since taking up office, my message is clear: the European Union wants Ukraine to win this war.
We will do whatever it takes for that. pic.twitter.com/swv1ZVAlyL
As winter begins Russia has also unleashed devastating barrages against Ukraine's power grid and on the frontline Ukraine's fatigued forces are losing ground to Moscow's grinding offensive.
"The situation in Ukraine is very, very grave," Ms Kallas, a former prime minister of Estonia, said. "But it's clear that it comes at a very high cost for Russia as well."
The new EU leaders - the bloc's top officials along with European Commission President Ursula von der Leyen - were set to hold talks with Ukraine's President Volodymyr Zelensky.
Mr Zelensky on Friday appeared to begin staking out his position ahead of any potential peace talks.
He called on NATO to offer guaranteed protections to parts of Ukraine controlled by Kyiv in order to "stop the hot stage of the war", and implied he would then be willing to wait to regain other territory seized by Russia.
"If we speak ceasefire, (we need) guarantees that Putin will not come back," Mr Zelensky told Sky News.
Ms Kallas said that "the strongest security guarantee is NATO membership."
"We need to definitely discuss this - if Ukraine decides to draw the line somewhere then how can we secure peace so that Putin doesn't go any further," she said.
From day one of the war, the EU has stood by the side of Ukraine.
— António Costa (@eucopresident) December 1, 2024
From day one of our mandate, we are reaffirming our unwavering support to the Ukrainian people.
Proud to be in Kyiv with @kajakallas and @Marta1Kos. pic.twitter.com/hlBxXCVVh7
Diplomats at NATO say there appears little prospect of the alliance granting Ukraine membership soon given opposition from a raft of members cautious of getting dragged into war with Russia.
Ms Kallas said the EU "shouldn't really rule out anything" in terms of the question of sending European troops to help enforce any ceasefire.
"We should have this strategic ambiguity around this," she said.
Mr Trump has cast doubt on continuing the US's vast aid for Ukraine and called on EU countries to do more.
Ms Kallas said the EU would use a "transactional language" to try to convince Mr Trump that backing Ukraine was in the interest of the US.
"Aid for Ukraine is not charity," she said. "A victory for Russia definitely emboldens China, Iran, North Korea."
The new EU foreign policy chief said the bloc would continue seeking to put Ukraine in the "strongest" position - if and when Ukraine chose it was time to negotiate with Russia.
But she conceded that it was becoming "increasingly difficult" for the 27-nation bloc to agree on new ways to ramp up support for Ukraine.
"This war has been going on for quite some time and it is harder and harder to explain it to our own people," she said. "But I don't see any option."
