KUALA LUMPUR (Reuters) - Pirates opened fire at a Japanese cargo ship off the coast of Somalia, but the vessel managed to survive the hijacking attempt, the International Maritime Bureau said on Saturday.

Already this week, a record four ships have been seized in this vital sea lane linking Asia and Europe in some of the world's most dangerous waters, sparking fears piracy there could worsen.

Noel Choong, head of the bureau's piracy reporting centre in Kuala Lumpur, said pirates in two speed boats chased the Japanese-operated general cargo ship, with 20 crew on board, in the Gulf of Aden earlier in the day.

"The pirates fired from these boats as they tried to board the ship," he said, but added that the ship managed to survive the attempted hijacking.

No casualties were reported, but the vessel was slightly damaged due to bullet holes, he said.

Piracy is rife off Somalia, which has been mired in anarchy since warlords overthrew a dictator in 1991. Maritime officials say at least 30 ships have been hijacked off the coast of the Horn of Africa nation so far this year.

Most of them brought ransoms of at least $10,000, and in some cases much more. A lot of tha