United States negotiators have raised Ukraine's potential withdrawal from its eastern region of Donbas in exchange for joining NATO, Ukrainian President Volodymyr Zelenskyy said.
The proposal came following the latest round of peace talks with Russia in Abu Dhabi.
Zelenskyy reiterated his position that Kyiv was not willing to retreat from its territory.
But he said that if Ukraine received a comprehensive security package, that includes security guarantees, a strong and well-funded Ukrainian army and funds to rebuild, he would put the question to a referendum.
"The Americans raised a question: 'What if Ukraine joins NATO? Would Ukraine withdraw from Donbas?' This was the first time this had been raised," Zelenskyy told reporters in remarks released by his office on Saturday.
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"That is why I believe NATO is back on the agenda. But you have also heard [Russian President Vladimir] Putin's signals, and this is an extremely difficult conversation."
The developments came after Ukraine fired almost 1,200 drones at Russia in one of its biggest aerial attacks since the Russian full-scale invasion nearly five years ago.
Russia's defence ministry said its forces downed 1,198 Ukrainian long-range drones between late Thursday and early Friday.
However, strikes on three Russian regions killed seven people, including a 12-year-old, and wounded 17, local officials said.
Ukraine's General Staff said it hit an oil refinery in the Russian city of Volgograd, starting a fire.
Russia launched 117 drones against Ukraine, killing at least two people including a 17-year-old, authorities said.
A single Russian attack on Friday on southern Zaporizhzhia hit two five-storey apartment blocks and wounded 30 people, including six children, officials said.
The war has killed hundreds of thousands of soldiers and thousands of civilians, analysts and humanitarian organisations say.
Russian attacks on Ukraine's energy infrastructure caused widespread power outages across about half of the country's regions on Friday, first deputy minister of energy Artem Nekrasov said.
Meanwhile, US and Russian negotiators raised the stakes in talks to stop the fighting by including for discussion wider European security arrangements that have been in place since the end of the Cold War.
"Russia wants to talk about questions of European security architecture. We don't need to talk about that. There is one," German Chancellor Friedrich Merz said, referring to the NATO military alliance.
After two days of talks with Russian and Ukrainian delegations in Abu Dhabi this week, US envoys went in person to brief Russian President Vladimir Putin in the Kremlin and Zelenskyy, who was meeting with European leaders in Dublin, Ireland.
More talks are planned as negotiators seek a breakthrough in peace efforts, according to US envoys Steve Witkoff and Jared Kushner.
Kushner said Russia has agreed to include European countries in upcoming talks.
Europe's presence at the negotiating table will allow negotiators "to address the broader postwar European security arrangements alongside the territorial issues between Ukraine and Russia," Kushner said on X.
As European leaders scrambled to find an envoy to the talks, they were also wary of Russian manoeuvring.
The European Union's top diplomat Kaja Kallas said the bloc must insist on a ceasefire before taking the negotiations further.
"If we start to talk about what Russia is asking regarding the security architecture, we actually walk into the traps that Russia is setting us," she said in Dublin on the sidelines of an EU summit.
Five countries — Germany, France, the United Kingdom, Italy and Poland — together with the EU institutions will represent Europe in possible peace talks between Russia and Ukraine, European Council president António Costa said on Friday evening.
"Europe must speak with one clear voice and united voice in all discussion that shape the future of Ukraine and the security of our continent. And this is ultimately a team effort," Costa, who chairs EU summits, told a press conference at the end of the meeting in Dublin.
- With reporting by Deutsche Presse-Agentur
