Adds details and comments from Starmer's spokesman in paragraphs 5-8

LONDON, Feb 17 (Reuters) - British Prime Minister Keir Starmer said on Monday his country's national security was facing a generational challenge and that it was crucial for all of Europe to spend more on defence.

The call comes as Starmer and other senior European leaders prepare to meet in Paris to discuss their response to U.S. President Donald Trump's decision to begin talks to end Ukraine's almost three-year war with Russia.

"We're facing a generational challenge when it comes to national security," Starmer told reporters before heading to Paris for an emergency leaders' meeting.

"I think there's a bigger piece here as well, which is that this isn't just about the front line in Ukraine. It's the front line of Europe and of the United Kingdom. It's about our national security and I think that we need to do more."

Starmer has said he is prepared to put British peacekeeping troops on the ground in Ukraine if there is a deal to end the war with Russia.

But his spokesman indicated that Britain may only send troops if the U.S. provided some security guarantees.

The nature of U.S. support will be discussed when Starmer and Trump meet in Washington next week, the spokesman said.

"U.S. support will remain critical and a U.S. security guarantee is essential for a lasting peace," he said.

Starmer said Europe needed to spend more because the ongoing conflict with Russia affected the whole of the continent, not just Ukraine.

"We need to step up in terms of our collective response in Europe. And by that, I mean capability, by that I mean playing our full part when it comes to the defence of the sovereignty of Ukraine if there's a peace agreement.

"I want the UK and all European allies to step up, and for the UK to play a leading part in that."

(Reporting by William James, Sam Tabahriti and Andrew MacAskill;
Writing by Catarina Demony;
Editing by Kate Holton and Sachin Ravikumar)

((catarina.demony@thomsonreuters.com; +44 (0) 7900 276927 (Twitter: @CatarinaDemony);))