Image caption The US says it has raised questions over this missile launch vehicle with China

China's president pledged strong ties with North Korea, amid serious tensions between the two Koreas in the wake of Pyongyang's failed rocket launch.

Hu Jintao's promise came at a meeting with a top Workers' Party delegation leader in Beijing on Monday.

It came as North Korea threatened "unprecedented action" against Seoul.

Meanwhile the US says it has raised allegations with China that a missile launcher seen in Pyongyang last week was of Chinese origin.

Tension on the Korean peninsula is high following the failed rocket launch on 13 April. Pyongyang said it was putting a satellite into orbit but critics said the launch was a disguised test of missile technology banned under UN resolutions.

South Korea also says there are signs that North Korea is planning a third nuclear test.

North Korea had almost completed preparations for the test which will take place ''soon'', an unidentified source told Reuters.

This source, the news agency said, has correctly predicted past events, including North Korea's first nuclear test in 2006.

'Peace and stability'

China - which is North Korea's closest ally and biggest trading partner - did not block UN condemnation of the launch.

But in the meeting on Monday with the Workers' Party international relations chief, Kim Yong-il, Mr Hu emphasised close ties between the two nations.

Image caption President Hu (r) said China and North Korea will work for peace and stability on the Korean peninsula

"We will carry on this tradition... boost strategic communication and coordination on key international issues and work for peace and stability on the Korean peninsula," state television quoted him as saying.

China - seen as the country with the greatest degree of influence over North Korea - has repeatedly called for calm on the Korean peninsula, as tensions between the two Koreas have risen.

On Monday North Korea warned of "unprecedented" action against South Korea's ruling establishment, in response to its criticism of the rocket launch.

A special operation to begin "soon" would "reduce its target to ashes", the North Korean military said in an unusually strong statement. On Friday North Korea also held a rally calling for the death of South Korea's president.

South Korea, meanwhile, said last week it had deployed new missiles capable of hitting any target in North Korea.

Vehicle probe

Meanwhile, White House spokesman Jay Carney said in a briefing on Monday that Washington had raised questions over a mobile missile launch vehicle seen in a North Korean military parade earlier this month.

Analysts believe that the 16-wheel missile transporter-erector-launcher (TEL) seen carrying what appeared to be a new missile may have been of Chinese origin.

Last week Jane's Defence Weekly, citing an unnamed official, reported that the UN Security Council was investigating the claims.

If it had supplied the technology or vehicle, China could be violating UN resolutions passed after North Korean nuclear and missile tests in 2006 and 2009, the report suggested.

Asked about the matter, White House spokesman Jay Carney said the US would "continue to work with the international community, including China, to enforce sanctions against North Korea's ballistic missile program and nuclear programme".

"We've raised the allegations with the Chinese government ... as part of our ongoing close consultations on North Korea," he said.

China says it has abided by UN sanctions on North Korea.