The drill on Tuesday involved more than 100 soldiers along the Yalu river that marks the border between the two countries, South Korea's Yonhap news agency said.

The exercise sparked speculation China is readying itself against possible regime collapse in Pyongyang, it said.

Soldiers wearing orange life vests could be seen building pontoon bridges at various points along the river, possibly preparing to enter the North's territory should an exodus occur.

Each bridge stretched 20 to 30 metres (66 to 98 feet) , made up of six or seven sections connected to each other, the report said.

Yonhap quoted residents in the Chinese border city of Dandong as saying such exercises take place on a regular basis and usually in the summer.

Tens of thousands of North Koreans have fled poverty and repression in their homeland, almost all of them across the border to China, which regards the fugitives as economic migrants and repatriates them.

The report, based on Japanese government sources, is the most strident of recent claims that China has been involved in helping to arm its wayward ally after earlier allegations Beijing supplied technology.

Four giant trucks capable of transporting and launching ballistic missiles were exported by a Chinese firm last August, the leading Asahi Shimbun said.

The vehicles were likely those on display at the huge military display in April marking the centenary of the birth of the state's founder Kim Il Sung, the Asahi said.

The sale of weapons systems to Pyongyang is banned under UN Security Council resolutions aimed at containing the hermit state's nuclear ambitions.

But at Washington's urging, Tokyo and Seoul have avoided confronting Beijing in a bid to keep North Korea's patron onside in the international effort to tamp down tensions on the peninsula, the paper reported.

Top government spokesman Osamu Fujimura declined to confirm the report, saying it was "a matter of intelligence".

But he added, in general terms "if Japan obtains information about any violation of the UNSC resolution, the government will address the matter in cooperation with concerned nations."

Japanese government papers reveal four 16-wheel vehicles were transported aboard a Cambodian-registered ship, which was tracked by spy satellites leaving Shanghai on August 1 and arriving at Nampho in western North Korea three days later, the Asahi said.

The vessel then moved on to Osaka where the Japanese coastguard conducted an on-board inspection and discovered documents detailing the export of the vehicles, issued by an agent in Shanghai, the Asahi said.

A report was passed on to the Japanese government's intelligence office, the Asahi said.

The Asahi said four WS-51200 transport vehicles with a length of 21 metres (about 69 feet) were exported by a subsidiary of the China Aerospace Science and Technology Corp., which is affiliated to the Chinese military.

Vehicles of this kind are used in China to transport ballistic missiles and the 16-wheeler 51200 model was likely developed to transport the Dongfeng 31 intercontinental ballistic missile, the report said.

Japan, South Korea and the US agreed the shipment was a violation of UN Security Council Resolution 1874, which bans the export of weapons to North Korea, the paper said.

Washington raised the issue with Beijing in April, the paper reported. Chinese officials acknowledged the export had taken place, but said the vehicles were for civilian purposes, including the transportation of timber.

Two months ago the United States said it believed China's assurances that it was abiding by sanctions on North Korea after charges that Beijing supplied missile launcher technology.

"China has provided repeated assurances that it's complying fully with both Resolution 1718 as well as 1874. We're not presently aware of any UN probe into this matter," State Department spokesman Mark Toner told reporters.

"I think we take them at their word," Toner said at the time.