Andrew Mwangura, East Africa's Coordinator of Seafarers Assistance Program (SAP) said the crew on the Chinese MV Zhen Hua-4 had locked themselves in their cabins and radioed for help.

"Pirates boarded but 30 crew locked themselves in accommodation room and radioed piracy center in Malaysia," Mwangura said in a statement to Xinhua.

"They notified coalition forces who sent two helicopters and one warship. They fired on pirates but did not kill them. Pirates left ship after five hours so it was not hijacked," he said.

The attack came a day after the UN authorized troops to pursue the bandits on land in a bid to tackle the increasing problem of piracy in the Gulf of Aden.

The Chinese boat was one of four vessels attacked by pirates since Tuesday. Mwangura said a yacht, an Indonesian tugboat and a Turkish cargo ship were seized. The regional maritime official said the Turkish ship MV Bosphorus Prodigy has 11 crew on board.

Piracy off the coast of Somalia this year has earned gunmen millions of U.S. dollars in ransom and hiked shipping insurance costs.

The seizures have prompted some of the world's biggest shipping firms to switch routes from the Suez Canal and send cargo vessels around southern Africa instead -- which could push up the cost of commodities and manufactured goods.