China vowed "all-out efforts" Tuesday to rescue a Chinese cargo ship hijacked in the Indian Ocean with 25 crew aboard, bringing to six the number of vessels currently in the hands of Somali pirates.

This is the second time that a Chinese ship has been captured by marauding sea bandits in the area, after a fishing boat and its 24-member crew were held hostage for three months before being released in February.

"We will continue to follow developments closely and make all-out efforts to rescue the hijacked ship and personnel," foreign ministry spokesman Ma Zhaoxu told reporters, without giving any details of China's rescue plan.

According to the European Union's anti-piracy naval mission, the ship was seized on Monday 550 nautical miles (1,000 kilometres) northeast of the Seychelles and 700 nautical miles off the pirate-ridden coast of Somalia.

A statement on the website of China's transport ministry said the vessel, called De Xin Hai, was carrying coal from South Africa to India with a 25-member crew.

EU naval spokesman John Harbour told AFP they were all Chinese nationals.

A spokesman for the ship's owner Qingdao Ocean Shipping Company, which is based in the eastern China city of Qingdao, also confirmed to AFP that the 25 seamen were all employees of the company.

"We still cannot get in touch with them," he said, refusing to give his name or any further details.

The hijacking of the Chinese ship brings to six the total number of vessels currently in the hands of Somali pirates.

Since last year a flotilla of foreign warships has been patrolling the Gulf of Aden near Somalia, one of the busiest maritime trade routes on the globe that has been plagued by piracy.

China sent three navy ships to the Somali coast in January to join these efforts to protect the region's shipping from pirates, and has previously come to the aid of at least one stricken vessel, state media reports have said.

A Chinese navy ship thwarted a pirate attack on a Liberian-flagged Italian merchant ship in February in the Gulf of Aden, state-run media reported then.

That same month, a Chinese vessel came close to being hijacked by pirates, but a Danish warship came to its rescue and stopped the assault.

According to maritime sources in the Indian Ocean, this is the first time bandits have boarded a merchant vessel east of the 60th meridian between the Seychelles and Maldives, indicating pirates are moving further afield.

Experts had warned in recent days that subsiding winds near the Seychelles had attracted pirates, who generally launch attacks from so-called "mother ships" with tiny skiffs.

Pirates focus most of their attacks on transitional weather periods between monsoons, when the seas are calm enough for them to operate. This year's summer season came later than last year's.

Ma urged Chinese ships on Tuesday to stay away from the waters around the Seychelles.

"The foreign ministry has… issued a special reminder reminding Chinese ships and personnel not to get close to relevant waters, in case of accidents or danger," he said.

According to the transport ministry statement, China has asked the International Maritime Bureau to mobilise warships located near where the vessel was hijacked to help in rescue efforts.

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