Somali pirates seized a ship carrying fertilizer from the U.S. in the Indian Ocean and a British-flagged chemical tanker in the heavily patrolled Gulf of Aden — the first merchant vessel to be hijacked in the gulf in nearly six months, officials said on Tuesday.

The hijackings on late Monday showed that pirates are relentless in their pursuit of quick money from ransoms and that ship owners need to take extra precaution when sailing in the Horn of Africa, said Noel Choong, who heads the International Maritime Bureau’s piracy reporting centre in Kuala Lumpur, Malaysia.

The waters off Somalia are teeming with pirates, who have hijacked dozens of ships for multimillion-dollar ransoms in the past two years. An international naval force now patrols the Gulf of Aden, one of the world’s busiest shipping lanes.

After the latest hijackings, pirates now hold 12 vessels and 263 crew members, Mr. Choong said. Pirates anchor their captured crafts near Somalia’s shore in the pirate strongholds of Haradhere and Hobyo. International forces can’t rescue the vessels without risking the lives of the crew, leaving negotiated ransoms as the only safe means of resolution.

The latest incidents brought the number of attacks in the Gulf of Aden and off Somalia to 214 this year, with 47 vessels hijacked, Mr. Choong said. That compares to 42 successful attacks out of 111 attempts in 2008, before the EU Naval Force deployed in the Gulf of Aden in December 2008.

The U.K.-flagged tanker St James Park was the first merchant vessel to have been hijacked in the Gulf of Aden in nearly six months, Mr. Choong said. He said the ship issued a distress message on Monday, seeking help after it was attacked.

The distress call was picked up by the Greek rescue and coordination centre in Piraeus, which in turn relayed the message to the International Maritime Bureau and other agencies, he said.

The maritime bureau could not establish communication with the vessel but was informed by the ship’s owner early Tuesday that the tanker has been hijacked, Mr. Choong said.

The spokesman for the European Union’s anti-piracy force, Cmdr. John Harbour, said the St. James Park was seized while in the Internationally Recognised Transit Corridor in the Gulf of Aden that is patrolled by the international naval coalition.

The St. James Park set sail from Tarragona, Spain, and was headed for Tha Phut, Thailand, he said. The tanker has 26 crew members from the Philippines, Russia, Georgia, Romania, Bulgaria, Ukraine, Poland, India and Turkey, Cmdr. Harbour said.

The ship was last reported to be heading toward the northern coast of Somalia, and the E.U. Naval Force was monitoring the situation, he added.

Pirates last hijacked a Yemeni fishing boat in the Gulf of Aden on Dec. 18, but the St James Park was the first merchant vessel to have been taken in the busy waterway since July 8, Choong said.

Three hours after the St James Park was hijacked a Panamanian—flagged carrier with 19 crew members was seized by pirates off the southern coast of Somalia. The ship is managed in Greece, he said.

Greece’s Merchant Marine Ministry said the Navios Apollon was carrying fertilizer from the United States to India. It was taken 240 nautical miles northeast of Seychelles, it said. The crew was comprised of one Greek and 18 Filipinos, it said.

In another development, pirates released the Singapore-flagged container ship Kota Wajar on Monday, the E.U. Naval Force said. The vessel was hijacked in mid-October in the Indian Ocean north of the Seychelles islands with a crew of 21 on board.

Somalia has not had an effective central government since 1991 as regional warlords vie for power, and impoverished young men have increasingly taken to piracy in recent years in hopes of a big ransom payoff.

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