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.

SALDANHA: Seized on February 22, 2009. The Maltese-flagged cargo ship, sailing to Slovenia, has 22 crew and was loaded with coal. TITAN: Seized on March 19, 2009. The 43,000 dwt Saint-Vincent-flagged cargo vessel with 24 crew was sailing from the Black Sea when it was attacked by pirates.

NIPAYIYA: Seized on March 25, 2009. The Greek-owned and Panama-registered MT Nipayia, was seized by pirates 450 miles from Somalia's south coast.

BOW ASIR: Seized on March 26, 2009. The 23,000-tonne Bahamas-registered chemical tanker was carrying caustic soda. It was operated by Salhus Shipping and carried 27 crew consisting of a Norwegian captain, 19 Filipinos, five Poles, one Russian and one Lithuanian.

INDIAN OCEAN EXPLORER: Seized March 2009: The 35-meter boat was built in Hamburg as an oceanographic research vessel. It accommodates around 12 passengers. The yacht Serenity with three people on board was seized in late February or early March.

HANSA STAVANGER - Seized April 4, 2009: The 20,000-tonne German container vessel was captured about 400 miles off the southern Somali port of Kismayu, between the Seychelles and Kenya. The vessel had a German captain, three Russians, two Ukrainians and 14 Filipinos on board.

TANIT: Seized April 4, 2009: The yacht was carrying two couples and a 3-year-old child. Pirates also seized a Yemeni tug.

MALASPINA CASTLE - Seized April 6, 2009: The 32,500-tonne bulker is UK-owned but operated by Italians.