Somali pirates have released a Bulgarian-flagged ship they seized in May, the Bulgarian foreign ministry said on Thursday.

The chemical tanker Panega with 15 Bulgarian crew was seized about 160 km (100 miles) east of the Yemeni port of Aden on May 11.

"The ship has been released this morning. We know that the 15 sailors are in good health," foreign ministry spokeswoman Vesela Cherneva said. She declined to comment on whether a ransom was paid.

Prime Minister Boiko Borisov said the ship was released with the help of the Bulgarian state and the Bulgarian maritime operator, Navibulgar, which owns Panega, but he did not elaborate, according to national radio.

Hijackings off the coast of Somalia, a chaotic Horn of Africa nation, have earned pirates tens of millions of dollars in ransoms, raised shipping insurance premiums and forced ships to take longer, costlier routes to dodge pirate hot spots.

In June, Somali pirates freed a British-flagged ship which had 25 crew, including eight Bulgarian sailors, after a ransom was paid.

At least 17 ships are being held by pirates. In the latest seizure on Sept. 8, a Maltese-flagged merchant vessel was attacked and hijacked inside the internationally recommended transit corridor in the Gulf of Aden. – Reuters