This could be a crucial week for the war in Ukraine, with two hastily arranged talks taking place in Paris and Riyadh.
European leaders are meeting in France as they scramble to start talks to end the conflict with Vladimir Putin on Donald Trump's plane.
Russian Foreign Minister Sergei Lavrov and US Secretary of State Marco Rubio are due to meet in the Saudi capital on Tuesday.
Ukraine is not taking part in any talks.
Russia launched a full-scale invasion of Ukraine in February 2022 and now controls more than a fifth of its territory, mainly in the south and east.
Harry Farley, London political correspondent
Sir Keir Starmer is hoping to bridge the gap between European leaders and the Trump White House's criticism of defence spending.
Starmer's proposal to deploy British troops in Ukraine is part of the role he wants to play.
The government has previously said the terms of any peace deal depend on Ukraine. That has changed with the new US administration indicating that a return to the 2014 border is "unrealistic".
Instead, Sir Keir will hope that more European countries will join him in Paris to secure a deal - and to provide their forces to deter a Russian attack.
But while the Prime Minister is in Paris, the debate in Westminster over how much the country should spend on defence is ongoing.
The Labour Party has pledged to "set out a path" to increase defence spending from the current 2.3% of GDP to 2.5%. Defense sources say it will be a significant increase.
But there is no date for when it will happen - and many argue that it is urgent now.
Germany correspondent Damien McGuinness in Berlin
A sign of how unsettled German leaders are about Trump's approach to Ukraine is that Chancellor Olaf Scholz is also in Paris just days before national elections.
All mainstream parties have condemned the American proposal for a peace deal without Ukraine or the EU. Politicians on the far-right and populist-left welcome talks with Putin and want to stop arms supplies to Kiev. But they will not come to power.
So, whatever the next German government, Berlin's support for Ukraine will remain strong. Because Berlin's political elites recognize that a bad deal - one that undermines Ukraine's sovereignty - would be disastrous for Germany.
But given Germany’s war-torn 20th century, voters here are wary of militarization.
The country has successfully distanced itself from Russian power over the past three years and has vastly increased defense spending. But that has hit the German economy hard, and the subsequent budget dispute has led to the collapse of the German government.
Politicians are therefore trying to avoid discussing difficult issues like increasing NATO’s military capabilities or German peacekeeping forces in Ukraine in public – at least until after the election.
Poland
Sarah Rainsford, Eastern Europe correspondent in Warsaw
Poland has been a key supporter of Ukraine since Russia’s full-scale invasion began and is a key supply point for military and humanitarian aid entering the country.
It is also a strong voice that Russia cannot be allowed to win the war it has started – because the security of all of Europe is at stake. So it is worrying that the United States seems to be giving in to Moscow’s main demands before the talks even begin, while Poland very clearly sees Russia as the aggressor and dangerous.
Poland spends heavily on its own military because of Russia - now up to about 5% of GDP - and agrees with the US that the rest of Europe should do the same.
On the way to the talks in Paris, Polish Prime Minister Donald Tusk wrote in X: "If we, Europeans, fail to spend big on defense now, we will be forced to spend 10 times more, if not prevent a larger war."
On the question of whether Polish troops would be sent to Ukraine - to help enforce any possible ceasefire - government officials are cautious, having ruled it out for now.
Nordic and Baltic countries
Nick Beek, Europe correspondent in Copenhagen
Denmark will be the only Nordic country at Monday's meeting. But European diplomats say it will also represent the interests of its Baltic neighbors to the east - Estonia, Latvia and Lithuania - all of whom border Russia and are seen as particularly vulnerable to any future attacks by Putin.
The shockwaves of Trump's second term are already reverberating around Denmark.
Trump's renewed desire to seize Greenland, an autonomous Danish-dependent territory, prompted a whistle-stop tour of European allies last month to drum up support for Danish Prime Minister Mette Frederiksen.