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.

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

"As far as I know, business people can enter and leave North Korea freely," added the official, who declined to give his name, without making clear which country had ordered the move.

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 the agency 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 several vehicles including cars, minibuses and lorries passing over the bridge crossing the Yalu river that marks the frontier, in both directions.

Passenger trains also crossed from each bank, while North Korean boats patrolled the river and a helicopter hovered overhead before returning to the Korean side, he said.

Much of the trade in Dandong is not illegal under United Nations sanctions, which mainly ban luxury goods and weapons-related items.

But China's commercial lifeline is seen as helping to thwart international efforts to pressure the isolated North Korean regime to change its ways.

China's total trade with North Korea stood at $1.31 billion in the first quarter, down 7.2 percent year-on-year, Customs said Wednesday. Exports were down 13.8 percent to $720 million, with imports up 2.5 percent to $590 million.

But economists find tracking trade flows between the two a challenge, and no further details or explanations were immediately available.

When asked about the drop at a regular press briefing on Wednesday, foreign ministry spokesman Hong Lei said that trade links were normal and positive.

"China and North Korea maintain normal state-to-state relations," he said. "The development of Chinese-North Korean relations is beneficial for the peace, stability and development of the region."

Hong acknowledged that tour groups had cancelled planned trips to North Korea but said the "border area remains normal".

Most travellers crossing the Dandong border are Chinese businessmen or North Koreans who work or have business in China, while many of the Chinese citizens who live in Dandong are ethnic Koreans.

Tourist traffic at the crossing is largely limited to Chinese groups, with most international visitors flying straight to Pyongyang.

Nicholas Bonner, who runs Beijing-based Koryo Tours, said his agency had received no notification of any restrictions. "We're still running tours," he told AFP, adding that the next one was planned for Saturday.

South Korea raised its military watch alert to "vital threat" Wednesday ahead of an expected North Korean missile launch, as UN chief Ban Ki-moon warned the situation on the Korean peninsula may be slipping out of control.