FACTBOX: Ships held by Somali pirates

(Reuters) - A U.S. Navy destroyer put pressure on Somali pirates on Thursday to free an American ship captain taken hostage in the first seizure of U.S. citizens by the increasingly bold sea gangs.

Gunmen briefly hijacked the Maersk Alabama container ship on Wednesday, but the crew retook control after a confrontation in the Indian Ocean, where pirates have attacked five vessels in the last eight days.

Pirates attacked at least 15 vessels off the coast of Somalia in March, after only two in January and February.

Below are facts about the ships believed to be under pirate control and about the increase in piracy:

STOLT STRENGTH: Seized November 10. The chemical tanker had 23 Filipino crew aboard. It was carrying nearly 24,000 tonnes of oil products.

CHEMSTAR VENUS: Seized November 15. The tanker was traveling from Dumai, Indonesia, to Ukraine. It had 18 Filipino and five South Korean crew.

NAMES UKNOWN: Seized on December 10. Pirates hijacked two Yemeni fishing vessels with a total of 22 crew in coastal waters in the Gulf of Aden. Five crew reportedly escaped.

NAMES UNKNOWN: Seized on December 16. A yacht with two on board, an Indonesian tugboat used by French oil company Total. Pirates also hijacked the Chinese fishing vessel Zhenhua-4 with 30 Chinese crew aboard but it was freed the next day.

LONGCHAMP: Seized on January 29, 2009. The liquefied petroleum gas tanker, built in 1990, had 13 crew on board, 12 Filipinos and one Indonesian. The tanker has a capacity of 3,415 tonnes.