North Korea's Kim Jong-un arrives in Beijing for talks ahead of possible U.S. summit

North Korean leader Kim Jong-un was expected to meet Tuesday with China's president at the start of a visit to Beijing that's believed to be an effort to co-ordinate with his only major ally ahead of a possible second summit with U.S. President Donald Trump.

A long motorcade including motorcycle outriders reserved for state leaders left a Beijing train station on Tuesday morning shortly after the arrival of an armoured train consisting of 20 to 25 cars most of whose windows were blacked-out along tracks lined by police and paramilitary troops.

Chinese Foreign Ministry spokesperson Lu Kang gave no details about Kim's schedule or China's role as an intermediary between the U.S. and North Korea. But he said further information about Kim's activities, the outcome of his meetings and a possible visit by Chinese President Xi Jinping to North Korea would be "released in due course."

Kim held three summits last year with Xi, his most important ally, before and after summits with Trump and South Korean President Moon Jae-in.

Kim and Trump pledged to work towarddenuclearization at their landmark June summit in Singapore, but the agreement was short on specifics. Negotiations have made little headway since.

"Kim is eager to remind the Trump administration that he does have diplomatic and economic options besides what Washington and Seoul can offer," Harry J. Kazianis, director of defence studies at the U.S.-based Centre for the National Interest, said in an e-mailed statement.

"In fact, during his New Year's Day speech, Kim's 'new way' that he referred to may well have been a veiled threat to move closer to Beijing. That should make America quite concerned."

Kim left for China on a private train on Monday afternoon accompanied by his wife, Ri Sol-ju, and senior North Korean officials, including Kim Yong-chol, a key negotiator in talks with the United States, and foreign minister Ri Yong-ho, North Korea's state-run KCNA news agency said.

China's official XinhuaNews Agency confirmed that Kim was visiting from Monday to Thursday at Xi's invitation.

The visit coincides with what South Korean officials say is Kim's 35th birthday on Tuesday.

Kim's visit to North Korea's most important economic and diplomatic ally follows reports of advanced negotiations for a second summit between Washington and Pyongyang aimed at resolving the standoff over North Korea's nuclear and missile programs.

Kim said in his New Year's speech he was ready to meet Trump any time to achieve their common goal of denuclearization of the Korean peninsula. But he warned that he may seek an alternative path if U.S. sanctions and pressure against the country continued.

In an interview with CNBC on Monday, U.S. Secretary of State Mike Pompeo praised China's support for resolving the North Korean crisis and said he did not think the U.S. trade dispute with China would affect this.

Kim's visit comes as a U.S. delegation opened a second day of talks with Chinese counterparts in Beijing aimed at ending the trade war between the world's two largest economies.

Ties between China and North Korea, which had frayed as Pyongyang stepped up its missile and nuclear tests up to late 2017, seem to have warmed over the past year, as Kim engaged with China, South Korea and the United States.