China is North Korea's largest trading partner and a vital source of diplomatic, economic and political support for the isolated nuclear state.

Train journeys between the East Asian neighbours were halted in 2020 under strict border closures during the pandemic, but resumed on Thursday.

The K28 train from Pyongyang arrived in Beijing three minutes ahead of schedule, at 8:37 am (0037 GMT) on Friday, a Beijing Railway information officer told AFP.

A day earlier, South Korea's Yonhap news agency reported that a train had been seen crossing the Sino-Korean Friendship Bridge over the Yalu River.

The river separates North Korea's Sinuiju from northeastern China's Dandong.

AFP journalists aboard the K27 train departing Beijing and bound for Pyongyang on Thursday saw certain carriages were reserved only for passengers travelling to North Korea.

Those wagons are then attached to another train in Dandong to transport them across the border to Sinuiju, tour agencies told AFP.

There, the wagons -- as well as North Korean domestic carriages -- are attached to a third train heading to Pyongyang.