Ukraine's allies will seek to "strengthen" a US plan to end the war with Russia when they meet at the G20 summit in South Africa, UK PM Sir Keir Starmer has said.

The summit begins a day after President Volodymyr Zelensky warned Ukraine faced "one of the most difficult moments in our history" over pressure to accept the plan - leaked details of which have been seen as favourable to Moscow.

Zelensky held phone talks with Sir Keir and the leaders of France and Germany on Friday. Afterwards, the PM said Ukraine's "friends and partners" remained committed to securing a "lasting peace once and for all".

Neither US President Donald Trump nor Russian President Vladimir Putin are attending the G20.

The widely leaked US peace plan includes proposals that Kyiv had previously ruled out: ceding eastern areas it now controls, significantly cutting its army size, and pledging not to join Nato.

Washington has been pressing Kyiv to accept, and sent senior Pentagon officials to the Ukraine earlier this week to discuss the proposals.

But there is nervousness in Europe over what has been perceived as a set of terms heavily slanted in Moscow's favour. EU foreign policy chief Kaja Kallas called the prospect of it being adopted a "very dangerous moment".

According to Reuters news agency, she told reporters: "We all want this war to end, but how it ends matters. Russia has no legal right whatsoever to any concessions from the country it invaded, ultimately the terms of any agreement are for Ukraine to decide."

Ahead of the Ukraine talks at the G20, Sir Keir said gathered leaders would "discuss the current proposal on the table, and in support of President Trump's push for peace, look at how we can strengthen this plan for the next phase of negotiations".

He continued: "Not a day has passed in this war where Ukraine hasn't called for Russia to end its illegal invasion, roll back its tanks and lay down its guns.

"Ukraine has been ready to negotiate for months, while Russia has stalled and continued its murderous rampage.

"That is why we must all work together, with both the US and Ukraine, to secure a just and lasting peace once and for all."

As part of the White House's plan, Ukraine would be obliged to cut the size of its army and pledge not to join the Nato military alliance, a long-held Kremlin demand.

Speaking on Friday, Trump warned that Ukraine would lose more territory to Russia "in a short amount of time" and that Zelensky "is going to have to approve" the plan.

Trump said he had given Ukraine until Thursday to agree to the plan, which he described as an "appropriate" deadline.

Ukraine relies on deliveries of US-manufactured advances weaponry to arms its forces, including air defence systems.

Kyiv has also been dependent on intelligence provided by Washington since Russia launched its full-scale invasion of Ukraine in 2022.

During a meeting with his security cabinet on Friday, Putin confirmed the US had presented its proposed peace plan, and said it could be the "basis" for a settlement - though added detailed talks on its terms had not yet been held in the Kremlin.

He said Russia was willing to "show flexibility" but was also prepared to fight on.

In a 10-minute address in front of the presidential office in Kyiv, Zelensky warned that Ukraine would face "a lot of pressure... to weaken us, to divide us".

"We're not making loud statements," he went on, "we'll be calmly working with America and all the partners... offering alternatives" to the proposed peace plan.

The White House has pushed back on claims that Ukraine was frozen out of the drafting of the proposal.

An unnamed US official told CBS News, the BBC's US media partner, that the plan was drawn up "immediately" following discussions with Ukraine's top security official Rustem Umerov, who agreed to the majority of it.

The leaked draft proposes Ukrainian troops' withdrawal from the part of the eastern Donetsk region that they currently control, and de facto Russian control of Donetsk, as well as the neighbouring Luhansk region and the southern Crimea peninsula annexed by Russia in 2014.

Russia currently controls about 20% of Ukrainian territory and its troops have been making slow advances along the vast front line, despite reported heavy losses.

Kyiv would receive "reliable security guarantees", the plan says, although no details have been given.

The document says "it is expected" that Russia will not invade its neighbours and that Nato will not expand further.

The draft also suggests Russia will be "reintegrated into the global economy", through the lifting of sanctions and by inviting Russia to rejoin the G7 group of the world's most powerful countries - making it the G8 again.