FACTBOX: Ships held by Somali pirates

(Reuters) - Somali pirates seized a U.S.-flagged, Danish-owned container ship on Wednesday in the latest of a sharp rise in attacks off the Horn of Africa nation, officials said.

There have been at least 15 pirate attacks on vessels off the coast of Somalia which were reported by the International Maritime Bureau for March, after only two in January and February. Earlier hijackings focused on the Gulf of Aden.

Below are facts about the ships believed to be still held by pirates as well as the increase in piracy:

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

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.

MAERSK ALABAMA - Seized April 8, 2009: The 17,000 ton Danish container ship was hijacked about 500 km (300 miles) off Somalia and was carrying 20 U.S. sailors.

* PIRACY: KEY FACTS:

-- In 2008 there were 293 incidents of piracy against ships worldwide -- 11 percent up on the year before. Attacks off Somalia and in the Gulf of Aden increased nearly 200 percent.

-- All types of vessels have been targeted. The pirates boarding the vessels were also better armed than in previous years and prepared to assault and injure the crew.

Sources: Reuters/International Maritime Bureau Piracy Reporting Center/Lloyds List/Inquirer.net