Jeremy Corbyn has urged Donald Trump and Kim Jong Un to "calm down" amidst escalating tensions between the US and North Korea.
The Labour leader said you "cannot play fast and loose with nuclear weapons", adding that Britain had a role to play in bringing the two leaders "back from the brink".
US President Trump has warned the North Korean regime to "get their act together" or face extraordinary trouble, suggesting he had been too mild when he vowed to unleash "fire and fury" on North Korea if it continued to threaten the US.
Speaking to the Press Association in Bristol, Mr Corbyn said: "We cannot play fast and loose with nuclear weapons and nuclear threats because do you know what - a nuclear explosion doesn't stop at national borders, it doesn't stop at the vicinity where the bomb drops."
The Labour leader, a long time anti-war campaigner, said the result of a nuclear strike would go on for decades, adding: "I ask them both, calm down.
"There are phone calls that could be made, there's discussions that could be held.
"Surely in the interests of sanity and safety over the whole world, do it."
Mr Corbyn said Britain and other nations with nuclear weapons also had a role to play in reducing the tensions.
"All the five declared nuclear weapons states, which includes the United States, Russia and China, as well as Britain and France, need to get on board on this to bring North Korea back from the brink, bring Donald Trump back from the brink, and support the NPT (the treaty on the non-proliferation of nuclear weapons) and support the UN's efforts to make nuclear weapons illegal worldwide," he said.
North Korea has laid out plans to strike the US island territory and major military hub of Guam in the Pacific.
It comes after US intelligence analysts concluded that North Korean scientists had developed a nuclear warhead small enough to fit on a ballistic missile.
The UN Security Council last weekend backed a punishing new round of sanctions against North Korea following the test-firing of two intercontinental ballistic missiles, which experts believe could be capable of hitting the US mainland.
Koji Tsuruoka, the Japanese ambassador to the UK, told BBC Radio 4's Today programme: "If there is any provocation, it all comes from North Korea.
"If they were to agree to conduct themselves in a constructive manner according to international rules there will be no use for any rhetoric that should be deployed.
"The problem comes from North Korea and it is totally wrong to say that other responses are propagating them."
On Thursday, First Secretary of State Damian Green said it is "obviously" in Britain's interests that the stand-off between Washington and Pyongyang does not lead to conflict.
He added that the "sensible" way to proceed is to step up international pressure on the regime of President Kim through the United Nations.