North Korea's secretive leader Kim Jong-Il is believed to have begun a much-anticipated trip to ally China aboard a special train on Monday, South Korea's Yonhap news agency reported.

The agency, quoting sources in Seoul and Beijing, said a 17-carriage train crossed the Yalu border river to Dandong at 5:20 am Monday Chinese time (2120 GMT Sunday) amid tight security.

It was thought likely to be headed to the eastern Chinese port city of Dalian en route to Beijing, Yonhap said.

A South Korean intelligence official confirmed to AFP that a North Korean train arrived in Dandong early Monday. "But we're still trying to check if Kim Jong-Il was aboard."

Analysts say that if the trip is confirmed, China could press Kim to return to the six-party nuclear disarmament talks that it chairs, in return for badly needed aid.

China, the North's sole major ally and its main source of food and fuel, is seen as among the few nations able to put pressure on Pyongyang's hardline regime.

But the mysterious sinking of a South Korean warship in late March has clouded prospects for nuclear negotiations, amid growing suspicions a North Korean torpedo attack was to blame.

"We have confirmed the arrival of a special train at Dandong, and we believe it is highly likely that Chairman Kim is on board," Yonhap quoted a Seoul official as saying, referring to the North Korean leader.

Around 200 Chinese police surrounded the station in Dandong, Yonhap said.

It quoted a Beijing diplomatic source as saying the train was headed for Dalian, a journey of about four hours from Dandong.

It was unclear why Dalian would be the first stop, but the North could be seeking to learn more about port operations. It is trying to develop its own port at Rajin on the border with China and Russia.

Kim, who is said to dislike air travel, has visited China four times since 2000 by train. The last trip in January 2006 was shrouded in secrecy until it had ended.

North Korea suffers persistent food shortages, aggravated by a bungled currency reform last November.

"Kim will likely express his commitment to returning to six-party talks while leaving a date for the return up to host China," said Yang Moo-Jin of Seoul's University of North Korean Studies.

"In return, Kim will receive economic aid from China."

The talks, which the North quit in April 2009, grouped the two Koreas, China, Russia, the United States and Japan.

US and South Korean officials have indicated they cannot restart under a cloud of suspicion about the North's involvement in the sinking of the warship Cheonan near the disputed inter-Korean border with the loss of 46 lives.

The South has not so far directly accused the North, which denies responsibility. The South's President Lee Myung-Bak raised the issue Friday in talks with Chinese President Hu Jintao in Shanghai.

Yang told AFP Kim "will try to convince the Chinese leadership that South Korea fabricated the Cheonan incident to set up North Korea.

"If China is convinced, it will promise to cooperate with the North in the possible future UN handling of this case. Otherwise, China will take a prudent stance without siding with North Korea."

Chung Mong-Joon, head of South Korea's conservative ruling party, said it was "disappointing and worrying" that Kim appeared to be visiting China now.

"The Chinese leadership should clearly convey our worry and anger to Chairman Kim over the Cheonan incident, on the occasion of his trip to China," Chung said.