A merchant ship sailing off Somalia has been hijacked by unknown raiders, Spain’s defence ministry said on Tuesday, fuelling fears pirates are returning to the Gulf of Aden and Red Sea waters already plagued by a surge in attacks on shipping.

A Spanish warship rushed to check on the Maltese-flagged vessel Ruen on Friday after reports it had been hijacked. It “has been under piracy (control) since the morning of Dec. 14,” the ministry said in a statement, in its first confirmation of the ship’s fate.

“The MV Ruen has indeed been hijacked. With regard to the questions on who the hijackers are, the planned boarding of the vessel … we do not have that information,” the ministry added in an emailed response.

Some maritime security sources said their assessment was that the incident was the first hijacking of a merchant ship by Somali pirates since 2017.

Pirates who caused chaos in the key waterways from 2008 to 2018, may have returned, possibly encouraged by a relaxation of security, or taking advantage of the chaos caused by attacks on shipping by Yemen’s Iran-aligned Houthi group amid the war in Gaza.

“They clearly think they have a permissive environment ashore and success begets success,” said Gerry Northwood, a former captain who commanded warships in the region with Britain’s Royal Navy.

“Add to which naval patrols are thin on the ground and security on ships is almost non-existent,” said Northwood, a consultant with maritime security company MAST.