European leaders are set to travel to Washington on Monday where they are expected to urge Donald Trump not to bend to Vladimir Putin's demands.
It comes after Mr Putin's meeting with Mr Trump last week, during which he is said to have demanded Ukrainian concessions of land in order for there to be a ceasefire.
Sir Keir Starmer will visit the White House alongside French, German, Italian, EU and Nato leaders, in order to support Volodymyr Zelensky and call for a harder stance against Russia.
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It is understood Mr Putin is demanding Ukraine surrender the entire Donbas region, which consists of the Donetsk and Luhansk areas of eastern Ukraine.
The Trump administration has insisted a discussion regarding the mineral-rich region is needed, adding that "both sides" will need to make compromises.
On Sunday, French President Emmanuel Macron said the group's will was to "present a united front between Europeans and Ukrainians".
He warned the US President: "There is only one state that proposes a peace that would be a capitulation: Russia.
PICTURED: Sir Keir Starmer and Volodymyr Zelensky met in the garden on 10 Downing Street earlier this month
|REUTERS
"If we are weak with Moscow, we are preparing for tomorrow's conflicts."
While they have not explicitly said Ukraine needs to remain in control of Donetsk, European leaders have declared it necessary for Mr Zelensky to decide the fate of the territory.
Although the Ukrainian President has rejected calls for the surrender of the entire region, he has accepted the possibility for negotiations with Mr Putin directly, saying any territorial changes should be made during a three-way meeting alongside the US.
Mr Zelensky said: "We need real negotiations which means they can start where the frontline is now.
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PICTURED: French President Emmanuel Macron attended a video conference with members of the 'Coalition of the Willing'
|REUTERS
"The contact line is the best line for talking and the Europeans support this."
The continued suggestions for face-to-face talks between Mr Putin and Mr Zelensky, which have been consistently rejected by Russia, is thought to be playing a role in the wider European strategy aimed at showing that Russia is the obstacle to peace.
European leaders have praised the White House after Mr Trump promised security guarantees to a Western-backed peacekeeping force in Ukraine once the fighting has stopped.
Sir Keir and Mr Macron are leading the "Coalition of the Willing", a group of 30 nations aiming to help keep Ukraine free if a peace deal is signed.
Vladimir Putin and Donald Trump met in Anchorage, Alaska last week
|REUTERS
The plans would involve European forces, including Royal Air Force fighter jets, and British military trainers and engineers would be sent to Ukraine in order to support the country's military.
For months, European leaders have been calling for Mr Trump to provide a form of security "backstop" in order to ensure Russia will not attack again.
Over the weekend, some leaders publicly thanked the US for supposedly offering "Nato Article five-like" guarantees during a call which briefed them on the summit with Putin.
Article five is a clause which determines that an attack on one Nato member is considered an attack on all.
However, No10 has asked the Trump administration to clarify what security guarantees it is offering, and Mr Macron said he would question "to what extent" the US is ready to contribute.