ABUJA, Feb 13 (Reuters) - Pirates shot dead the captain and the chief engineer on a cargo ship off the coast of Nigeria on Monday, an International Maritime Bureau (IMB) official said, the latest in a string of attacks on vessels off the coast of Africa's No. 1 oil producer.

"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. Due to the continuous firing the captain and the chief engineer were shot," a notice on the IMB website said.

The attack. about 110 miles south of the commercial hub and main Nigerian port Lagos, comes two days after pirates on two boats in the same stretch of water fired shots at a different cargo vessel and chased it for 25 minutes before giving up, the website said.

Pirates off the coast of Nigeria tend to raid ships for cash and cargo rather than hijacking the crews for ransom like their counterparts off the coast of Somalia.

The frequency of attacks, while not as high as off the Somali coast or surrounding Indian Ocean, is on the rise. (Reporting by Joe Brock; Editing by Louise Ireland)