Berlin - The fate of five Germans held by pirates off the coast of Somalia remained unclear Monday, hours after the captain of a US-operated cargo ship was freed from Somali hostage takers in a dramatic US Navy rescue operation.

Germany's Foreign Ministry was intensively trying to resolve the hijacking of the Hansa Stavanger freight ship, a spokeswoman said on Monday. She could not give further details of negotiations taking place.

A total of 24 sailors are on board the container ship. The captain and four naval officers are German, Germany's Der Spiegel news magazine reported.

The pirates had phoned the German vessel's owners, Leonhardt & Blumberg in Hamburg, and set out ransom demands, according to Der Spiegel.

The cargo vessel was captured by pirates on April 4, around 400 nautical miles off the Somali coast.

More than 12 ships, holding more than 220 people, are in the grips of pirates. Most recently, pirates on Saturday seized the Italian tugboat Buccaneer.