North Korean leader Kim Jong-un is seeking an ice-breaking trip to key ally Beijing in September to meet China's outgoing and new leaders, a source with ties to Pyongyang and Beijing told Reuters on Friday.

The planned visit comes amid signs of a thaw in Pyongyang as it desperately seeks investment to lift its impoverished and isolated economy using Chinese money.

The trip was requested during this month's visit to Beijing by Kim's uncle, Jang Song-thaek, effectively the second most powerful figure in North Korea.

"It will be a get-to-know-you trip," said the source who has correctly predicted events in the past, including North Korea's first nuclear test in 2006 days before it was conducted, as well Jang's rise.

"It will be a state visit. This was one of the most important missions of Jang Song-thaek's visit," the source added.

North Korea relies heavily on China for economic and diplomatic support and the trip would be Kim's first abroad since becoming leader.

Jang met Chinese President Hu Jintao and Premier Wen Jiabao on his trip to Beijing.

It is not known whether Kim Jong-un, the 20-something leader of North Korea who took power in December following the death of his father, Kim Jong-il, visited China before assuming office.

Kim Jong-il last visited China in May 2011.

China's foreign ministry had no immediate reply when reached by telephone and asked to comment on the visit.

NUCLEAR THREAT

Although Kim Jong-un appears to have changed his father's austere style -- he has staged a rock concert with Disney characters in Pyongyang -- North Korea still regularly hurls bellicose rhetoric at South Korea.

Experts believe the North is closing fast on the technical capacity to stage a third nuclear test and the source said Pyongyang stood by its longstanding requirement the United States sign a formal treaty to end the 1950-53 Korean War and to give diplomatic recognition as the price for not staging a test.

The war ended in an armistice.

"There is no doubt North Korea has the capability, but China is strongly opposed to it," the source with ties to Pyongyang and Beijing said.

"North Korea wants a permanent peace treaty to replace the armistice in exchange for dropping plans for a third nuclear test. It's been 60 years and time to (formally) end the war with a peace treaty," the source added.

Siegfried Hecker, a U.S. nuclear expert who has visited the North's main Yongbyon nuclear facility four times since 2004 and was the last foreign expert to visit the site in late 2010, wrote in a report published on August 6 that the North could be technically ready for a test within two weeks.

Hecker and co-author Frank Pabian wrote that North Korea might conduct a simultaneous test using plutonium and highly enriched uranium.

The North's previous tests have used plutonium and the use of highly enriched uranium would give Pyongyang a second route to a nuclear weapon.

"Whether and when North Korea conducts another nuclear test will depend on how high a political cost Pyongyang is willing to bear," Hecker and Pabian wrote.

"Beijing has continued to expand aid and trade with North Korea, but has also applied significant diplomatic pressure on Pyongyang not to test," they said.