Brussels: Pirates hijacked a Japanese ship carrying 20 Filipino sailors in the Somali Basin, the European Union's anti-piracy mission said on Monday.

A Danish warship was dispatched to investigate after the owners of the Panama-flagged merchant vessel Izumi received a distress signal Sunday indicating it was likely under pirate attack, the EU naval force said.

"In the early hours of October 11 the captain of the vessel made contact with the Danish warship, stating that pirates were in charge of the MV Izumi," the EU mission said in a statement.

The French warship FS Floreal is monitoring the pirated vessel, which was 170 nautical miles south of Mogadishu, the statement said.

Japanese officials said the multi-purpose ship, owned by Japanese shipping company NYK-Hinode Line Ltd., had been surrounded by the pirates off Kenya's coastal city of Mombasa.

The ship was carrying steel products and was scheduled to travel from Kimitsu port in suburban Tokyo to Singapore and Mombasa, the shipping company said in a statement.

After boarding the Izumi, pirates were travelling in the direction of Somalia, Japan's Jiji Press reported, citing sources from the transport ministry.

"We don't know who the pirates are nor where they are from," said foreign ministry official Yasumasa Iijima, adding that contacts with Izumi were lost after the urgent message Sunday.

Japanese officials said the ship weighs 14,162 tonnes but the EU mission reported a deadweight of 20,170 tonnes.

The EU mission said last week that Somali pirates were holding 17 ships with 369 hostages.

The latest hijacking came as Japan increased efforts to counter piracy in international waters.

Japan last year joined the United States, China and more than 20 other countries in the maritime operation against pirates who have attacked ships in the waters off the Horn of Africa, a key route leading to the Suez Canal.

Tokyo has also dispatched two maritime surveillance aircraft and scores of military personnel to the region to beef up its anti-piracy mission.