Pirates have made another attempt to hijack a ship, this time north of Somalia in the Gulf of Aden, the Nato Shipping Centre reports.

The agency said on Friday that the Motor Vessel Ryu Gyong, a bulk carrier sailing under the North Korean flag, was attacked late on Thursday east of Mukalla, Yemen.

"The vessel witnessed a pirate mother ship launch two skiffs," the shipping centre said. "The skiffs and mother ship followed the Ryu Gyong for approximately two nautical miles, firing automatic weapons and a rocket-propelled grenade."

The ship is one of six which have been unsuccessfully attacked by pirates since the beginning of April, according to reports from the Nato centre and the Piracy Reporting Centre of the International Maritime Bureau. One is the Maersk Alabama, the American vessel which was initially seized by pirates but is now back under the control of its crew.

The Nato centre said the Ryu Gyong had been threatened four times off the east coast of Somalia between March 20 and 24. It had been approached by pirates twice and was attacked twice.

The centre's website on Friday listed 18 vessels, including dhows and a yacht, which are currently under the control of pirates. Most are being held on the east coast of Somalia. The majority have been seized since early March, but some were hijacked as long ago as last August.