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 to prevent the coronavirus from spreading.

State news agency Xinhua said the first train left Dandong, a Chinese city bordering North Korea, bound for Pyongyang on Thursday morning.

It quoted an unidentified China Railways executive as saying that the train service would "serve as both a vital window for cross-border travellers and a dynamic link strengthening the friendship between these two nations".

China Railway said in a separate statement regular train services would also resume between Beijing and the North Korean capital on Thursday evening.

The resumption of the train link symbolised a return to a stronger bilateral relationship, said Lim Tai Wei, a professor and East Asia expert at Japan's Soka University.

It signalled greater access to "the largest trading nation on Earth" for North Korea, Lim told AFP, while it was also important for China's "periphery diplomacy".

Beijing has been a crucial lifeline for North Korea's moribund economy.

China has fully reopened its borders since the pandemic, but North Korea has proceeded more slowly. Direct flights and train services with Russia resumed last year.

While the resumption suggests a "re-normalisation" of contact between China and North Korea, it does not necessarily mean increased support from Beijing, said Associate Professor Chong Ja Ian from the National University of Singapore.

"A lot of the previous limit on contact seems to be due to Pyongyang's apprehensions about broader contact, which have diminished," Chong told AFP.

- Change trains -

The K27 train due to depart on Thursday would make a few stops, including at the port city of Tianjin, and then head northeast to Dandong.

Wagons holding passengers bound for Pyongyang will then be attached to another train there, taking them across the border to the nearby North Korean city of Sinuiju, said Rowan Beard from Young Pioneer Tours, a company specialising in North Korea travel.

Beard said those wagons, as well as North Korean domestic carriages, would be attached to a new train that would then head to Pyongyang, where China Railway said it would arrive on Friday evening.

Trains will run in both directions between Beijing and Pyongyang every Monday, Wednesday, Thursday and Saturday, China Railway said.

The Dandong-Pyongyang service would operate daily, it said.

Travel agents for an official ticketing booth in Beijing told AFP on Tuesday that anyone with a valid visa can now buy train tickets to the North.

That includes Chinese people working and studying in North Korea, as well as North Koreans working, studying and visiting family abroad.

Entry and exit procedures would be completed at the Dandong border crossing and at Sinuiju in North Korea, China Railway said.

Tickets are currently available for offline purchase in several Chinese cities, it added.