Pirates have abandoned a Turkish-flagged bulk carrier they hijacked while it was en route to the Kenyan port of Mombasa, leaving its crew of 25 unharmed, the head of a regional maritime body said.

Yasin C was seized on Wednesday 250 miles east of Mombasa.

"Yasin C was abandoned yesterday. The pirates abandoned it, and I think the crew will seek aid from the navy before coming to Mombasa," Andrew Mwangura of the East African Seafarers' Assistance Programme told Reuters. "All the 25 crew were unharmed." He said the vessel is yet to arrive in Mombasa.

Fatih Kabal, a spokesman for Bergen Shipping which operated the ship, told Turkey's state-run news agency Anatolian the 22,353 tonnes vessel had been retrieved. The ship had been on its way from the Black sea to Kenya.

"The ship's captain gave the good news that the pirates had abandoned the ship," Kabal said. The crew had locked themselves in the engine room until they realised the pirates had gone.

Over the last few years sea gangs have seized dozens of ships in the Indian Ocean and Gulf of Aden.

Despite international naval patrols, pirate activity is expected to increase in coming months as the weather improves.

Mwangura did not say why the pirates abandoned the Turkish vessel. This can happen if ships develop mechanical problems or run out of fuel.

Separately, the U.S. Navy said it captured six suspected pirates in the Gulf of Aden after they opened fire on a navy vessel with small arms from their boat.

"USS Ashland fired two rounds at the skiff from her .... 25-mm gun. The skiff caught fire and the suspected pirates abandoned the skiff. The Ashland deployed her rigid-hull inflatable boats ... to assist the pirates who were in the water near their skiff," a navy statement said.

On Sunday, Somali pirates seized a South Korean oil tanker, the 300,000-tonne Samho Dream, as it sailed to the United States from Iraq. The ship is now in Somali waters.