EPA/ShutterstockUkraine's President Volodymr Zelensky has accused Russia of "doing everything it can" to prevent a meeting with Vladimir Putin to try to end the war.
US President Donald Trump has sought to bring the two leaders together, but he said on Friday "that's like oil and vinegar... they don't get along too well".
Russian Foreign Minister Sergei Lavrov said Putin was ready to meet Ukraine's leader "when the agenda is ready for a summit, and this agenda is not ready at all", accusing Zelensky of saying "no to everything".
After an intensive week of diplomacy, in which Trump first met Putin in Alaska and then Zelensky with European leaders in Washington, the US president said the war was turning out to be the most difficult he had tried to stop.
Trump said after a call with the Russian leader on Monday that he had begun arrangements for a Putin-Zelensky summit that he would join afterwards.
Ukraine's president has backed the move, but he has sought security guarantees from Western allies to prevent any future Russian attack in the event of a peace deal: "Ukraine, unlike Russia, is not afraid of any meetings between leaders."
On a visit to Kyiv, Nato Secretary General Mark Rutte said Trump was aiming to "break the deadlock" and the alliance was working on robust security guarantees with the US and Europe to ensure Putin "will never ever try to attack Ukraine again".
Speaking alongside Rutte, Zelensky said he wanted Ukraine's security guarantees to reflect Nato's Article 5, which considers an attack on one member of the alliance an attack against all Nato members.
"This is the beginning of a big undertaking, and it is not easy, because guarantees consist of what our partners can give Ukraine, as well as what the Ukrainian army should be like, and where we can find opportunities for the army to maintain its strength," Zelensky said.
Rutte said the alliance was working with Ukraine to define the guarantees, explaining that they would focus on making Ukraine's military as strong as possible and involve Western security commitments. It was "too early to exactly say what will be the outcome", he added.
Russia's foreign minister appeared to dent hopes of any potential summit, telling NBC News that "there is no meeting planned".
Sergei Lavrov said Russia had agreed to show flexibility on a number of issues raised by Trump at the US-Russia summit in Alaska last week.
He went on to accuse Ukraine of not showing the same flexibility in subsequent talks in Washington, blaming Ukraine for hindering progress toward a peace deal.
Lavrov said it was "very clear to everybody that there are several principles which Washington believes must be accepted".
He said this included no Nato membership for Ukraine and discussions of territorial issues: "Zelensky said no to everything," Lavrov said.
He was speaking after EU foreign policy chief Kaja Kallas told the BBC that Putin was seeking territorial concessions from Ukraine that were a "trap that Putin wants us to walk into".
"We are forgetting that Russia has not made one single concession and they are the ones who are the aggressor here," Kallas said.
Despite latest efforts to broker a peace deal, Russia launched one of its heaviest attacks on Ukraine in weeks on Thursday, launching 574 drone and 40 missiles in one night.
Telegram/MadyarMeanwhile, a Ukrainian drone blew up an oil pumping station in the Russian region of Bryansk, halting oil deliveries along the Druzhba pipeline to Hungary and Slovakia - the third attack on the pipeline in nine days.
Hungary and Slovakia are largely dependent on the Druzbha pipeline for their oil supplies, and Budapest says it could take at least five days before operations resume. The two EU member states have complained to the European Commission.
The European Union sought to cut Russia's energy supplies after its full-scale invasion of Ukraine in 2022 and aims to phase out Russian oil and gas by the end of 2027.
Hungarian Prime Minister Viktor Orban wrote to President Trump to complain about the attack on the pipeline, and his officials posted what they said was Trump's handwritten response.
Facebook"Viktor - I do not like hearing this - I am very angry about it. Tell Slovakia."
"You are my great friend," he added.
The White House did not immediately respond to a request for comment.
