Somali pirates are believed to have hijacked Vietnamese owned bulk carrier with 24 crew members, approximately 520 nautical miles southeast of the port of Muscat, Oman, EU anti-piracy taskforce said on Thursday.

EU Naval Force spokesman Paddy O'Kennedy did not say when the 22,835-tonne MV Hoang Son Sun was seized with all Vietnamese nationals.

"No further details of the attack are known at this stage," O' Kennedy said in a statement.

He said the Mongolian flagged vessel was not registered with the Maritime Security Center- Horn of Africa (MSCHOA), which provides a service to mariners in the Gulf of Aden, the Somali Basin and off the Horn of Africa MSC (HOA) and had not reported to UK Maritime Trade Operations (UKMTO).

There are now 30 vessels and 724 hostages being held by Somali pirates.

The world piracy body (IMB) reported early in the week that the number of pirate attacks against ships rose in each of the past four years.

Ships reported 445 attacks in 2010, up 10 percent from 2009. While 188 crew members were taken hostage in 2006, 1,050 were taken in 2009 and 1,181 in 2010.

According to IMB, hijackings off the coast of Somalia accounted for 92 percent of all ship seizures last year, with 49 vessels hijacked and 1,016 crew members taken hostages.