The biggest border crossing between North Korea and China has been closed to tourist groups, a Chinese official said Wednesday as nuclear tensions mounted, but business travel was still allowed.

An official at the Dandong Border Office, who declined to give his name, told AFP: "Travel agencies are not allowed to take tourist groups to go there, since the North Korean government is now asking foreign people to leave."

"As far as I know, business people can enter and leave North Korea freely," he added.

China is North Korea's sole major ally and the provider of the vast majority of its trade and aid, with most of the business passing through Dandong.

A woman surnamed Wu at a travel agency in the town said municipal authorities told it on Tuesday that because of tensions in Pyongyang, Dandong travel firms would not be able to take tours into North Korea from Wednesday.

"It was absolutely North Korea's (decision) because the travel bureau told us 'North Korea is now no longer allowing tour groups to be taken in'," the woman said.

An AFP photographer at the border Wednesday saw cars and a larger vehicle passing over the bridge crossing the Yalu river that marks the frontier, in both directions.

On Tuesday Pyongyang advised foreigners to consider leaving South Korea, warning that the Korean peninsula was headed for "thermo-nuclear" war.

Last week North Korean authorities also warned embassies in Pyongyang to consider evacuating as it would be unable to guarantee diplomats' safety in the event of conflict, but the statement was largely dismissed as empty rhetoric.

Most governments made it clear they had no plans to withdraw personnel.

The Korean peninsula has been locked in a cycle of escalating military tensions since the North's third nuclear test in February, which drew toughened U.N. sanctions.

Pyongyang's bellicose rhetoric has reached fever pitch in recent weeks, with near-daily threats of attacks on U.S. military bases and South Korea in response to ongoing South Korean-U.S. military exercises. (AFP)