Athens/London - Pirates have hijacked two chemical tankers off the coast of Somalia over the past two days, the International Maritime Bureau (IMB) said in London on Thursday.

IMB spokesman Cyrus Mody said a Panama-flagged ship, was seized on Wednesday, while a second ship, which is registered in the Bahamas, was taken on Thursday.

The Greek Ministry of Merchant Marine in Athens said the 9,000-ton Panama-flagged Nipayia is owned by Greek shipping company Lotus Shipping and had 19 crew members. It was attacked 720 kilometres off Somalia as it was leaving Madagascar and was heading to the Persian Gulf.

Officials at the ministry said the crew members were in good health and that Lotus Shipping was currently in ransom negotiations with the pirates.

The Greek Minister of Merchant Marine Anastasis Papaligouras called the attack a "serious threat to the shipping industry."

The second vessel, the 23,000-ton, Norwegian-owned Bow Asir, was seized 400 kilometres east off the Somali coast on Thursday morning, according to the reports. The vessel, that has a crew of 27 and a Russian captain, was reportedly attacked by 20 pirates.

It was not clear what type of cargo the two ships were carrying.

The attacks occurred as more than 20 ships from the European Union, NATO, China and Russia patrolled the area in an effort to prevent attacks on sea lanes around the Horn of Africa.

Shipping routes off the coasts of Eastern and Western Africa have become increasingly dangerous in recent months, especially in the Gulf of Aden, Gulf of Guinea, off the coast of Cameroon and Nigeria.

Dozens of ships seized by pirates have been released only after owners paid tens of millions of dollars in ransom. (dpa)