Britain's top military chief has fired a warning shot to the West to not be cowered by Vladimir Putin.

In a rare and hard-hitting intervention, Admiral Sir Tony Radakin - the head of Britain's Armed Forces - urged NATO to be 'assertive in every domain', from nuclear weapons to cyberspace, as Donald Trump gears up for a high-stakes one-on-one with the Russian president in Alaska tomorrow.

Sir Tony's comments come amid fears that Trump could cut a peace deal over Ukraine that sidelines Kyiv's president, Volodymyr Zelensky.

But the US president has insisted he won't be pushed around, declaring Putin 'wouldn't mess with me' and promising swift action - either a breakthrough or more sanctions - if talks collapse.

Speaking from the Oval Office, Trump vowed to 'set the table for the next meeting' and warned: 'If it's a bad meeting, it will end very quickly, and if it's a good meeting, we will end up having peace in the very near future'.