Beijing: North Korean leader Kim Jong-un arrived in China on Tuesday on an unannounced visit, in an apparent effort to co-ordinate with his only major ally ahead of a possible second summit with US President Donald Trump over denuclearisation of the Korean peninsula.

Kim is visiting China from January 7 to 10 on the invitation of President  Xi Jinping, state-run Xinhua news agency reported on Tuesday.

He is expected to celebrate his 36th birthday here on Tuesday.

According to reports, he was accompanied by his wife, Ri Sol-ju, and senior officials, including Kim Yong-chol, his right-hand man in diplomatic negotiations with the United States and South Korea.

Kim's visit came only days after he warned of taking an alternative path if the United States does not ease sanctions and pressure on his isolated country.

The visit also comes amid reports that negotiations are under way for a second summit between Kim and Trump. The two met in June in Singapore for the first summit.

Kim visited China last year to meet Xi before and after his first meeting with Trump.

Speculation had grown on Monday that Kim was possibly making his way to China after South Korea's Yonhap news reported that a North Korean train had been seen crossing the border.

Both countries' media confirmed the visit on Tuesday morning when Kim's distinctive green and yellow train arrived at a station in Beijing.

His visit to Beijing, which is stated to be closest ally of Pyongyang, comes at a time when China itself is locked in a trade war with the US over Trump's demand to reduce the USD 375 billion trade deficit with the US.

Trump while seeking Xi's support in forcing Kim to relent and give up the nuclear and missile programme has also blamed the Chinese leader last year for tardy progress after his first summit with Kim at Singapore in June.

Since the Singapore summit, there is growing scepticism in the US over Kim delivering on his promise to do away with the nuclear programme despite Washington's assurance over North Korea's security.

Also, China and Russia -- both the permanent members of the UN Security Council -- have been putting pressure on the US to lift UN sanctions against North Korea.

But the US has been insisting on their continuance. US Secretary of State, Mike Pompeo insisted that sanctions should be enforced vigorously and without fail.

Observers say the outcome of Xi-Kim meeting would be watched for its likely impact on the planned second summit meet between Kim and Trump.

In his tweet in August last year Trump had said North Korea was "under tremendous pressure from China because of our major trade disputes with the Chinese government".

Significantly, Kim's visit is taking place when Chinese and US officials are holding talks to end the trade war.

The US and China have been locked in an escalating trade spat since early 2018, raising import tariffs on each other's goods.

Last year, Trump imposed tariff hikes of up to 25 per cent on USD 250 billions of Chinese goods. The move prompted China to increase tariffs on USD 110 billions of US goods.

Trump and Xi has agreed to halt any further tariff increases for 90 days beginning January 1.