A Somali pirate gang said it will kill 25 Chinese sailors aboard a hijacked ship if China makes good on a threat to launch a rescue attempt.

The De Xin Hai, carrying 76,000 tonnes of coal from South Africa to India, was captured 550 miles north-east of the Seychelles yesterday morning. Reuters quoted a gang associate in Haradheere, a noted piracy town on Somalia's east coast where the captured vessel may be headed, warning against any military operation to free the sailors.

"If they try that we will execute the whole crew … we tell them to change their mind regarding any rescue, otherwise they will regret it," said the man identified only as Hassan.

In Beijing, a foreign ministry spokesman, Ma Zhaoxu, earlier told reporters that the government was making "all-out efforts to rescue the hijacked ship and personnel". China has three warships patrolling the Gulf of Aden and Indian Ocean.

Though Chinese sailors have previously been held hostage on Taiwanese and Korean-flagged ships off Somalia, the De Xin Hai appeared to be the first Chinese vessel captured by pirates, according to Andrew Mwangura, head of the East African Seafarers' Assistance Programme.

He said a rescue attempt was unlikely because of the risk of crew casualties and the nature of the cargo. Coal carriers are vulnerable to tipping if "messed with", Mwangura said. "The gunmen are willing to die. The only way out of this is negotiation."

Somali pirates are now holding six vessels and 146 foreign crew. With the rough seas of the monsoon season becoming calmer, attacks are expected to increase sharply in number and geographical scope until April.

In the past few weeks pirate gangs using "mother ships" have moved into previously safe waters. One attack took place off the coast of Oman, to the north of Somalia, while the Chinese bulk carrier, which is owned by the Qingdao Ocean Shipping Company, was taken 700 miles south-east of the Somali coast.

There are up to 33 foreign warships on anti-piracy missions in the Gulf of Aden and Indian Ocean, but co-ordination among the different navies as well as shipping firms was still lacking, Mwangura said. The De Xin Hai, for example, was not registered with the Maritime Security Centre (Horn of Africa), which seeks to safeguard ships travelling in the region by tracking their positions.

"The Chinese navy is working one way, and Nato another," said Mwangura. "Meanwhile the pirates are expanding their area. It's very hard to stop them."