Somali pirates hijacked a Belgian-owned ship off Somalia on Saturday but a Dutch naval frigate foiled an attack on a Greek-owned chemical tanker in the Gulf of Aden.

The European Union's Maritime Security Centre reported that the 1,850-ton dredger, the Pompei, and its crew of 11 were en route to Durban, South Africa, when the ship was seized. "The vessel is now heading north-west towards Somalia at slow speed," the centre said.

The Pompei was sailing in a zone recommended by the European Union (EU) naval taskforce, more than 600 nautical miles east of the Somali coast. Pirates are now operating far further out to sea than previously.

The EU's security centre also reported that a new chemical tanker, the Handytankers Magic, was sailing through the Gulf of Aden to Suez when it was attacked by a pirate skiff.

"One rocket-propelled grenade was fired and landed on deck, but fortunately did not explode," the centre said. "The Dutch frigate, De Zeven Provincien... was in the vicinity and foiled the attack by stopping and boarding the attacking skiff."

The Dutch seized seven suspected pirates aboard the skiff, which was a Yemeni fishing vessel which had evidently itself been hijacked.