Reports said the Kang Nam 1, which was ordered to remain in Hong Kong after docking on Sunday, had been tailed by the USS Gary, a guided-missile frigate that docked on Saturday.

It was feared the 2,035-tonne general cargo ship had been carrying military supplies in breach of UN sanctions imposed on North Korea after its October 9 atom bomb test, the reports in Hong Kong and South Korea said.

But the head of the government's marine administration said the Kang Nam 1 had coincidentally docked while the US warship was making a routine port call.

"There's nothing sinister in this at all, it's all simply routine practice," Roger Tupper, the director of the Marine Department, told AFP.

"We had no tip off about military equipment. We simply selected the Kang Nam 1 on the same grounds that we would for any other vessel. It was all routine."

Tupper also denied that officials had stopped the ship under the terms of the UN sanctions. He said it was not unusual for North Korean vessels to stop in Hong Kong.

A spokesman for the United States consulate said it had no information on the ship and said the USS Gary was in Hong Kong on a port call, not to tail the North Korean ship.

"I can tell you that the USS Gary was not chasing any North Korean ships," spokesman Dale Kreisher told AFP.

The South Korean Yonhap news agency reported that the ship's crew members had said they would soon set sail.

"(We) came to pick up cargo. We will depart in two days after uploading the cargo," an unnamed sailor on the ship told Yonhap.

Tupper added the ship was being detained because it was in breach of a number of local safety regulations.

Reports in the local media and Lloyd's List, the London-based shipping directory, said officials had found 25 faults with safety and other equipment aboard the 22-crew vessel.

Lloyd's cited the acting assistant director at the Marine Department, Lee Kai-leung, as saying that 12 safety concerns were "detainable deficiencies mainly on life-saving and fire fighting appliances as well as navigational equipment including out-dated and obsolete charts".

UN sanctions were imposed on Pyongyang to prevent Kim Jong-il's hardline Stalinist regime from transferring or importing nuclear technology and to end the trade in contraband, such as drugs and illicit cigarettes, that is believed to help fund the North's nuclear programme.

The sanctions are part of a huge global diplomatic effort since the test to persuade North Korea to abandon its nuclear weapons programme.

A report in the South China Morning Post, Hong Kong's English-language daily, said the ship had arrived from Shanghai and was due to return home to Nampo, near Pyongyang, via Taiwan.

They said the captain, who would not give his name, had told reporters he was unaware of Pyongyang's first ever atom bomb test, or of subsequent UN sanctions.

China said it had no knowledge of the reports.

"I do not have information on that, but China will earnestly implement and abide by (UN) resolution 1718," Chinese foreign ministry spokesman Liu Jianchao told a regular press conference, after being asked about the detention of the ship.

"If there is any violation of the resolution found in Chinese territories, we will take necessary measures to fulfill our obligations."

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