N Korea's Kim Jong-un visits China's Xi Jinping

North Korean leader Kim Jong-un has arrived in Beijing for an unannounced visit, at the invitation of Chinese President Xi Jinping.

Kim will be in China until 10 January with his wife Ri Sol-ju, according to state media reports.

The visit comes amid reports that negotiations are under way for a second summit between Kim and US President Donald Trump.

The two met last June, the first such meeting for a sitting US president.

Speculation had grown on Monday that Kim was possibly making his way to China on board his armoured train.

The visit, during which he is being accompanied by several leading North Korean officials, is his fourth to China in less than a year.

Tuesday is also reportedly Kim's 35th birthday, though his date of birth has never been confirmed by Pyongyang.

China is an important diplomatic ally for North Korea, and one of its main sources of trade and aid.

"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 Defense Studies at the Centre for the National Interest told Reuters.

Kim, unusually, did not meet Mr Xi for the first six years of his leadership of North Korea.

But last year, he visited China three times. None of the trips were announced in advance.