(Adds Greek officials, China ship deal)

NAIROBI, Nov 22 (Reuters) - Pirates hijacked a Greek-owned bulk carrier on Thursday in the Gulf of Aden near Yemen, a Kenyan maritime official said on Sunday, but Greek officials said the attack may have been unsuccessful.

The vessel was taken 36 nautical miles off the Yemeni port of Balhaf and news of the seizure only emerged on Saturday, said Andrew Mwangura, coordinator of the Kenya-based East African Seafarers' Assistance Programme.

"Red Sea Spirit was taken by gunmen off the Yemeni coast last Thursday. She is flying the Panama flag," Mwangura said. "She is a Greek-owned bulk carrier."

However, a Greek merchant marine ministry spokesman said the managers of the ship, Sekur Holdings, did not confirm the incident. Sekur Holdings were not available for comment.

Meanwhile, pirates said they may release the Chinese De Xin Hai next week.

The Chinese bulk vessel was seized mid-October with 76,000 tonnes of coal and 25 Chinese crew. It is owned by the Qingdao Ocean Shipping Co.

"Negotiations between us and the owners to free the Chinese ship are going on now," a pirate named Mohamed told Reuters from the pirate stronghold of Haradheere. "We agreed on $3.5 million to free the ship."

Somali pirates have continued to defy foreign navies patrolling the waters off the Horn of Africa and are holding at least 13 vessels and more than 200 crew.

There was a pause in hijackings during monsoon rains, but the sea gangs have stepped up attacks in the past two months, extending their range to as far as the Seychelles, to evade the naval vessels.

Piracy attacks around the world numbered 324 during the year to Oct. 20, according to figures from the ICC International Maritime Bureau's Piracy Reporting Centre.

Attacks by Somali pirates in the Gulf of Aden and the east coast of Somalia, numbered 174, with 35 vessels hijacked and 587 crew taken hostage.

Nearly 20,000 ships pass through the Gulf of Aden each year, heading to and from the Suez Canal. (Reporting by Helen Nyambura-Mwaura in Nairobi, Mohamed Ahmed in Haradheere and Renee Maltezou in Athens) ((Email: nairobi.newsroom@reuters.com; tel: +254 20 222 4717)) (For Interactive factbox on Somalia please click on http://uk.reuters.com//news/factbox?fj=20090825142344.js&fn=Conflict%20and%20strife%20in%20bloody%20Somalia%20)

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