Pirates fired on a cargo vessel off the coast of Nigeria on Monday killing the captain and chief engineer, a maritime watchdog body said as it warned shipping to stay clear of the area.

"Armed pirates chased and fired upon a drifting bulk carrier. Vessel raised alarm and headed towards Lagos. All crew except the bridge team took shelter in the citadel," the International Maritime Bureau (IMB) Piracy Reporting Centre said in a statement.

Cyrus Mody, a manager at the IMB, said the vessel's captain and chief engineer "were killed in the shooting."

The attack, which took place 110 nautical miles off the coast, is one of three to have occurred in the area since Thursday, the IMB said.

The attackers are believed to be "Nigerian pirates", according to Noel Choong, head of the IMB's Kuala Lumpur-based piracy reporting centre.

He said the centre had alerted Nigerian authorities, who launched an operation to intercept the pirates and rescue the ship. He could not provide any information on the origins of the ship or its crew.

Nigeria's navy spokesman Kabir Aliyu confirmed the attack.

"MV FORCES was attacked by pirates at about 11O nautical miles south of Lagos Fairway Bouy. Two crew members seriously injured and later reported dead. Others sustained various degrees of injuries," he said in a statement to AFP.

The ship was sailing towards Lagos Fairway bouy, he said, adding that the rescue operation was being coordinated by state-run Nigeria Maritime Administration and Safety Agency and the navy maritime guard while its special naval joint operation in neighbouring Benin has also be alerted, he added.

"We are still searching for the vessel," he said.

The IMB said other recent attacks in the area included a tanker that was hijacked south of Nigeria on Thursday. Nigerian vessels intercepted that ship and rescued its crew.

The IMB, which is funded by shipowners, warned in September that the seas off Benin, Nigeria's neighbour, were emerging as a new piracy "hotspot" due to the weak enforcement capabilities of governments in the region.

West African leaders are scheduled to discuss the piracy issue at a regional summit that opens in Nigeria's capital on Wednesday.

Unlike the explosion of piracy off the coast of Somalia on the eastern side of the continent in recent years, those involved in the recent west African attacks have so far not appeared to be after ransom payments.

Fuel or oil cargo has been stolen for sale on the region's lucrative black market, while robberies have also occurred. Crew members have been beaten and the pirates tend to be heavily armed.

The theft of such cargo tends to be relatively sophisticated, with tankers often being directed to another pirate-controlled ship, where the fuel is transferred and then taken elsewhere for sale.