Dutch marines guarding a merchant ship have warded off an attack by pirates after it was accosted in the Arabian Sea south of Yemen, the Dutch defence ministry said Tuesday.

"Dutch marines warded off the attack when pirates approached the Flintstone", a ship that dumps rocks to protect oil and gas pipelines, at around 6:00 am (0500 GMT), it said in a statement.

The Flintstone's crew sheltered in a safe room as a fishing vessel launched a fast moving skiff towards the merchant vessel.

"The marines tried in vain to stop the fast-moving skiff, occupied by six men armed with an assortment of weapons including a rocket-propelled grenade (RPG), by firing tracers as a warning," the ministry said.

After one of the men aimed the RPG at the ship, the marines fired on the skiff, which turned around and joined the larger fishing vessel.

"It's not excluded that there may have been casualties among the attackers," the ministry said.

The Flintstone was underway from Singapore to Malta through the Suez Canal when the attack happened, it added.

Dutch marines teams of the Vessel Protection Detachment (VPD) are in certain instances put on board merchant ships on request from the ship's owners, the defence ministry said on its website.

Last year "heavily armed" VPD teams accompanied 11 ships through pirate infested waters, it added.