North Korea shut its borders in early 2020 to prevent the spread of Covid-19, and later bolstered defences along its northern boundary to deter its own nationals from re-entering the country illegally from China.

Several Western tour operators said last month they had sent small numbers of foreign travellers into the northeastern city of Rason after receiving permission from North Korean interlocutors.

But some of the companies said on Wednesday that those trips had been suspended for unclear reasons.

"We have been advised by our partners... that tours to Rason are currently paused," travel agency Young Pioneer Tours said in a notice on its website.

Fellow travel agency Koryo Tours said "Rason is temporarily closed", with Beijing-based general manager Simon Cockerell telling AFP that those already in North Korea would "finish their trips as planned".

Rayco Vega, tours coordinator for Spain-based travel agency KTG, also confirmed the suspension to AFP, adding: "We do not know the reason nor how long this will last".

North Korea has already welcomed some international trade and official delegations, and last year allowed Russian tourists to enter the country for the first time since the pandemic.

Rason, which lies near the borders with China and Russia, became North Korea's first special economic zone in 1991.

It has a separate visa regime and has been a testing ground for new economic policies, boasting the socialist country's first legal marketplace.

The vast majority of foreign visitors to North Korea before the pandemic were Chinese, though some US citizens also ventured there before Washington banned travel following the imprisonment and subsequent death of student Otto Warmbier in 2017.

People from South Korea, with which Pyongyang remains technically at war, are also barred from visiting without official permission.

China is a key ally and source of economic backing for North Korea's diplomatically isolated, UN-sanctioned government.

A Chinese firm that advertised a tour to Rason last month did not respond when asked by AFP to clarify if it had been affected by the suspension.