Jeremy Corbyn has urged Theresa May not to drag Britain into any US military action against North Korea.
The Labour leader has called on President Donald Trump and Kim Jong Un to "wind down the war of rhetoric".
Writing in the Sunday Mirror, Mr Corbyn said there can be "no question of blind loyalty to the erratic and belligerent Trump administration" over the crisis.
"Trump and Kim must immediately wind down the war of rhetoric, as the German Chancellor Angela Merkel has demanded," he said.
"The risks of an unintended escalation into full-blown conflict are too great for the whole world.
"We cannot play fast and loose with nuclear weapons and nuclear threats. Our government must press for measured responses to bring the temperature down.
"Our government must not drag our country into any military action over the Korea crisis, including joint exercises.
"There can be no question of blind loyalty to the erratic and belligerent Trump administration.
"US-led regime-change wars and the threat of more to come have made this crisis more dangerous and difficult to resolve."
Mr Corbyn said a Labour government would work to rid the world of nuclear weapons and added the "global pressure for dialogue and diplomacy must be overwhelming".
The Labour leader's intervention comes after China's President warned the US and North Korea not to exacerbate tensions after Mr Trump said Kim would "truly regret" military action.