South Korean police on Saturday stopped a planned launch of anti-North Korea leaflets across the border, an activist said, as Seoul seeks to open dialogue and defuse soaring tensions with Pyongyang.

The move came as the North issued a fresh threat to bomb any site where leaflets are launched as it accused Seoul of stirring the crisis which has engulfed the Korean peninsula ever since Pyongyang conducted a rocket test last December and a nuclear test in February.

The world is facing a "critical time", top U.S. diplomat John Kerry told China's President Xi Jinping on Saturday, citing tensions on the Korean peninsula, Iran's nuclear program and the conflict in Syria.

"Mr. President, this is obviously a critical time with some very challenging issues," Kerry told Xi in the Great Hall of the People in Beijing.

U.S. Secretary of State John Kerry Friday demanded North Korea abandon an expected missile launch as the communist state threatened a nuclear strike on Japan amid a chilling new evaluation of its offensive capability.

Kerry, visiting Seoul to give fulsome U.S. backing to military ally South Korea, joined President Barack Obama in decrying North Korea's incendiary rhetoric -- and urged China to step in.

The United States slammed North Korea's belligerence Thursday and pressed China to rein in its ally, as U.S. officials downplayed a chilling spy agency report that Pyongyang has a nuclear-armed missile.

The alarming assessment of the North's nuclear capabilities, revealed by a U.S. lawmaker at a congressional hearing, came as tensions on the Korean peninsula mounted over an expected missile launch by Pyongyang.

G8 foreign ministers on Thursday strongly condemned North Korea over its nuclear program.

After a two-day meeting in London, the ministers including U.S. Secretary of State John Kerry warned Pyongyang it faced further sanctions in the event of a missile launch that is expected imminently, amid soaring tensions on the Korean peninsula.

South Korea Thursday called for negotiations with North Korea on the future of the Kaesong joint industrial zone, which Pyongyang has threatened to shut down permanently after suspending operations.

"Normalization of the Kaesong industrial complex must be solved through dialogue," the South's Unification Minister Ryoo Kihl-Jae told reporters.

A powerful U.S. military sea-based radar is now in place to detect any possible missile launches by North Korea, according to a senior U.S. defense official.

"The SBX is in position," the defense official, who spoke on condition of anonymity, told Agence France Presse late Wednesday.

North Korea is "skating very close to a dangerous line" with its heated rhetoric and provocative actions, U.S. Defense Secretary Chuck Hagel told reporters Wednesday.

The United States and its allies hoped Pyongyang would tone down its inflammatory language but the American military was prepared for any possibility, Hagel said.

Russian Foreign Minister Sergei Lavrov warned Wednesday against heating up the North Korea crisis with military manoeuvres, but stressed that Moscow and Washington had a common stand.

"On North Korea we have no differences with the United States," he told journalists as he met with U.S. Secretary of State John Kerry for talks on the sidelines of a meeting of G8 foreign ministers in London.

German Foreign Minister Guido Westerwelle on Wednesday called on North Korea to stop its "war of rhetoric" and said the isolated communist state was "playing with fire".

South Korean and U.S. forces raised their alert status to "vital threat" Wednesday ahead of an expected North Korean missile test, with tensions wound tight during a five-day buildup to a key anniversary.