SEOUL, South Korea (AP) — South Korean media reported late Monday that North Korean leader Kim Jong Un may be on his way to Beijing for his fourth summit with Chinese leader Xi Jinping.

Officials in China and in Seoul had no immediate comment. North Korea rarely reports such visits until they are over.

The reports said a train like the one often used by Kim was seen crossing through the Chinese border city of Dandong late Monday amid heavy security.

South Korea's Yonhap News Agency speculated the train could be carrying a senior North Korean official, while the Hankyoreh newspaper cited sources as saying Kim was in China for a summit.

Yonhap said the train was expected to reach Beijing at about 10 a.m. Tuesday local time.

Kim used a very distinctive armored train made especially for him on visits to China last year.

Reports of the North Korean leader's possible trip to China come after U.S. and North Korean officials are believed to have met in Vietnam to discuss the location of a second summit between Kim and President Donald Trump.

Tuesday is also Kim Jong Un's birthday.

Kim started off his flurry of summits last year with a meeting with Xi. China is the North's most important trading partner and a key buffer against pressure from Washington.

If he is going to meet Xi, Kim could be hoping to coordinate his positions with China before the Trump summit.