Jeremy Corbyn has urged US and North Korean leaders to hold face-to-face talks “in the interest of sanity and safety”.
He was commenting on an escalating war of words between US President Donald Trump and the North Korean regime.
“We cannot play fast and loose with nuclear weapons and nuclear threats,” the Labour leader said.
Mr Trump has issued another warning to North Korea with a reference to US weapons being “locked and loaded”.
Speaking before Mr Trump’s latest tweet, Mr Corbyn said: “I think it’s time that both Donald Trump and Kim Jong-un toned it all down a bit.
“Maybe even spoke to each other. Maybe even returned to the table.”
He added: “I ask them both. Calm down. There are phone-calls that could be made, discussions that could be held.
“Surely, in the interest of sanity and safety over the whole world, do it.”
The Foreign Office repeated a statement it released two days ago when asked for a comment on Mr Trump’s latest intervention.
A spokesman said: “We will continue to work with the US and our international partners to maintain pressure on North Korea and counter the security threat posed by its pursuit of nuclear weapons.
“We have been consistently clear and forthright in our condemnation of North Korea’s destabilising and illegal behaviour, including through support for UN Security Council resolutions to bring in sanctions that will limit North Korea’s ability to pursue its nuclear weapons programme.”
North Korea has threatened to fire missiles into the US territory of Guam and the US president has warned North Korea of “fire and fury”.
Mr Trump has also boasted about America’s nuclear arsenal, while unconfirmed reports claim Pyongyang might have made a nuclear warhead small enough to fit onto a missile.
On Friday, Mr Trump tweeted: “Military solutions are now fully in place, locked and loaded, should North Korea act unwisely. Hopefully Kim Jong Un will find another path!”
The Labour leader said that “of course” the UK should play a role in mediating the current impasse.
“All the five declared nuclear weapons states need to get on board with this – to bring North Korea back from the brink – bring Donald Trump back from the brink.”
Mr Corbyn, a longstanding anti nuclear campaigner, said efforts should be made to “support the UN’s efforts to make nuclear weapons illegal worldwide.
“That surely has to be the way to go.”
Asked on Thursday about President Trump’s “fire and fury” warning, Damian Green, the UK’s first secretary of state, said: “Let’s hope it doesn’t come to that.”
He added: “That’s why we are operating as hard as we can through the proper diplomatic channels to make sure that we see a de-escalation of tensions.”
Tensions have increased in recent days, with North Korea saying a plan that could see it fire four missiles near Guam will be ready in a matter of days, if approved by Kim Jong-un.
It described President Trump as “bereft of reason”.
The US has warned the North its actions could mean the “end of its regime”.
US Defence Secretary Jim Mattis said Pyongyang would be “grossly overmatched” in any war against the US and its allies.