JOHANNESBURG — Ukraine’s Western allies will seek to reinforce a proposed U.S. plan aimed at ending the war with Russia as they gather for the G20 summit in South Africa, UK Prime Minister Sir Keir Starmer said on Saturday.

The summit opens amid heightened concern in Kyiv, where President Volodymyr Zelensky warned that the country is facing “one of the most difficult moments in our history” due to mounting pressure to accept the proposal — leaked details of which have been widely viewed as favorable to Moscow.

Zelensky held calls on Friday with Starmer and the leaders of France and Germany. Following the discussions, the UK prime minister said Ukraine’s partners remain committed to securing a “lasting peace once and for all.”

Neither U.S. President Donald Trump nor Russian President Vladimir Putin is attending the G20 summit.

Leaked elements of the U.S. proposal include concessions Kyiv had previously ruled out, such as withdrawing from parts of eastern territory currently under Ukrainian control.

Washington has been urging Kyiv to accept the plan, dispatching senior Pentagon officials to Ukraine earlier this week for talks.

The proposals have raised concern in Europe, with EU foreign policy chief Kaja Kallas describing the situation as a “very dangerous moment.”

She stressed that any settlement terms would be for Ukraine to decide and that Russia “has no legal right whatsoever to any concessions from the country it invaded.”

Ahead of the G20 discussions, Starmer said leaders would review the plan “in support of President Trump’s push for peace” and explore ways to “strengthen this plan for the next phase of negotiations.”

Zelensky has warned that Ukraine may soon face a difficult choice “between losing dignity or risking the loss of a key partner,” while insisting Kyiv would present alternatives to Washington’s draft.

Under the U.S. plan, Ukraine would significantly reduce the size of its army and pledge not to join NATO — a long-standing Kremlin demand.

Trump said on Friday that Ukraine would lose more territory “in a short amount of time” if it rejected the proposal, adding that he had given Kyiv until Thursday — Thanksgiving in the U.S. — to respond.

Moscow has confirmed receiving the draft. Putin said the plan could serve as a “basis” for negotiations but noted that detailed discussions had yet to be held.

He added that Russia was prepared to “show flexibility,” while also being ready to continue fighting.

The White House denied that Ukraine had been excluded from the plan’s development, with a U.S. official telling CBS News that it followed talks with Ukrainian National Security and Defense Council Secretary Rustem Umerov.

The leaked blueprint calls for Ukrainian forces to withdraw from the parts of Donetsk they still hold, effectively giving Russia full control of Donetsk, Luhansk, and Crimea.

In return, Kyiv would receive unspecified “reliable security guarantees,” while Russia would be gradually reintegrated into the global economy through the lifting of sanctions and the possibility of rejoining the G7 — returning it to a G8 format.

Russia currently controls around 20% of Ukrainian territory.

The political backdrop remains volatile. Russian strikes continue across eastern and western regions of Ukraine, with dozens killed in recent attacks. Meanwhile, Ukraine remains heavily reliant on U.S.-supplied advanced weaponry and intelligence support amid Russia’s slow advances along the front line.

U.S. Vice President JD Vance said on Friday it was a “fantasy” to believe that increased funding, weapons, or sanctions alone could deliver a Ukrainian victory, underscoring Washington’s push for a negotiated settlement. — BBC