Pirates armed with machine guns hijacked a Norwegian chemical tanker Thursday off the coast of Somalia, the ship's owner said, an attack that came less than 24 hours after a smaller Greek-owned vessel was seized in the same area.

The U.S. 5th Fleet, which patrols the pirate-infested Gulf of Aden, confirmed both hijackings and said they took place in the same area but separate from the gulf, one of the world's busiest — and now most treacherous — sea lanes.

The 23,000-ton Norwegian-owned Bow Asir was captured 250 miles off the Somali coast on Thursday morning, and the 9,000-ton Greek-owned Nipayia, with 19 crew members, was seized 450 miles off Somalia on Wednesday afternoon, the European Union's military spokesman said.

Both vessels are chemical tankers but their cargoes were not immediately made public.

Cmdr. Jane Campbell of the U.S. 5th fleet said both hijackings took place in a vast Indian Ocean expanse of over 750,000 square miles.

"This activity highlights the complexity of even trying to monitor an area this size," she said.

She said pirates also tried to hijack another Panamanian-flagged boat Wednesday but the crew fought off them off by speeding away and using fire hoses.

Norway's shipowners association said the Bow Asir had a crew of 27 with a Norwegian captain, but the 5th Fleet said there were 23 crew on board. Fleet spokesman Lt. Nate Christensen said the Norwegian ship was Bahamian-flagged.

A Nairobi-based diplomat said the Nipayia had 18 Filipinos on board and a Russian captain. He said the ship is managed by Athens-based Lotus Shipping, speaking on condition of anonymity because he was not authorized to speak to the press.

The owner of the Bow Asir, Salhus Shipping AS, said it received a security alert from the ship Thursday morning saying it was being chased by two small boats with suspected pirates. Sixteen minutes later, the ship's captain reported that pirates had boarded the vessel.

Three hours later, the shipping company received an e-mail from the Bow Asir confirming that 16 to 18 pirates carrying machine guns had gained control, managing director Per H. Hansen said in a statement.

"We have no reports of any injuries," he said. "We are doing our utmost to ensure the safety of the crew."

Late Thursday, Bow Asir was heading northwest in the direction of Somalia, according to the Norwegian Shipowners' Association.

"We have no information about any demands from the pirates yet," spokesman Haavard Aagesen. "Our main concern now is the crew members and their families."

Pirate attacks off the Somali coastline hit unprecedented levels in 2008, when pirates made 111 attacks and seized 42 vessels, mostly in the Gulf of Aden.

Seven ships have been seized so far this year, although there were roughly 10 times as many attacks in January and February 2009 as there was over the same period last year. There have been almost daily attacks in March.

Somalia has not had a functioning government since clan-based militias overthrew a socialist dictator in 1991 and then turned on each other.

Also Thursday, NATO announced its anti-piracy flotilla of five ships was resuming patrols off the Horn of Africa, joining at least 20 warships from the EU, the U.S., China, Russia and other navies that are trying to stop pirate attacks there.

Graeme Gibbon Brooks, founder of London-based private security company Dryad Maritime Intelligence, said the latest hijackings showed that the Somali pirates were moving their area of operations into the Indian Ocean.

"The coalition have put so much pressure on the Gulf of Aden that the pirates are popping up everywhere else," he said. "Because the area is that much bigger, it will be more difficult for the coalition to achieve the same amount of success as they have in the Gulf of Aden."