NAIROBI, Kenya — Crews on oil tankers aren't allowed to smoke above deck, much less carry guns, for fear of igniting the ship's payload. That's one of the main reasons Somali pirates met little resistance when they hijacked a U.S.-bound supertanker carrying $20 million in crude.

The Greek-flagged tanker — traveling from Saudi Arabia to New Orleans — had no escort when it was hijacked Sunday because naval warships are stretched too thin. The problem has been further exacerbated because pirates have expanded their operations to hundreds of miles out at sea.

The hijacking, one year after seizure of a Saudi supertanker led to heightened international efforts to fight piracy off the Horn of Africa, has highlighted the difficulty of keeping ships safe in the region — particularly oil tankers.

The Maran Centaurus was about 800 miles (1300 kilometers) off the coast of Somalia when it was hijacked with 28 crew, said Cmdr. John Harbour, a spokesman for the EU Naval Force. On Monday, it was headed toward Somalia's lawless coast, a location where pirates most likely will hold the vessel as they attempt to negotiate a multimillion-dollar ransom.

While some ships traveling in the region have been outfitted with high pressure water guns and piercing noisemakers to repel pirates, even this is shunned on oil tankers for fear of triggering a response from pirates armed with guns and rocket-propelled grenades.

If you're not allowed to smoke a cigarette on the upper deck of an oil tanker, why would you want someone with a weapon up there?" said Graeme Gibbon-Brooks, who heads the private security company Dryad Maritime Intelligence.

There is also the threat that an accident or gunfight could lead to a leak that would devastate thousands of miles of ocean or coastline.

Protecting the huge tankers that carry more than half of the world's oil supply is made even more difficult because of their slow speed.

Sailors can typically distinguish fishermen from pirates around 300 yards, but by then it is too late to stop most attacks, said Gibbon-Brooks.

Expense and legal worries rule out armed escorts on a separate ship, he said, suggesting the best way to evade attack was for tankers to increase speed and steer the ship 30 to 45 degrees either side of its course. The swinging motion increases wake, destabilizing pirate skiffs, and keeps the low stern moving where pirates usually board. Trailing nets or lines behind vessels can also help by fouling the propellers of pirate skiffs, he said.

The Maran Centaurus was traveling about 11 knots when it was hijacked, according to Maj. Marten Granberg of the EU Naval Force. Most successful hijacks occur on ships traveling less than 20 knots. The ship was heading toward New Orleans, he said.

Gibbon-Brooks said he had recently received information from contacts in Somalia that more attacks were planned on tankers.

Twenty percent of global shipping — including eight percent of global oil shipments — is funneled into the narrow, pirate-infested Gulf of Aden that leads through the Red Sea to the Suez Canal. The route is bordered on one side by the failed state of Somalia and on the other by the increasingly unstable country of Yemen.

The U.S. Energy Information Administration says about 5 percent of daily oil shipments pass down the east African coast and around South Africa's Cape of Good Hope, which is where the Maran Centaurus appeared to be headed, Granberg said.

The Maran Centaurus is carrying around 275,000 metric tons of crude, said Stavros Hadzigrigoris, from the ship's owners Maran Tankers Management. At current market rates the oil would be worth just more than $20 million.

The ship had 9 Greeks, 16 Filipinos, 2 Ukrainians, and a Romanian aboard. Granberg said the ship's owner reported the crew was not injured in the attack.

The vessel is only the second oil tanker captured by Somali pirates. Its seizure resurrected the fears raised by the takeover of the Saudi-owned Sirius Star, a hijacking resolved with a $3 million ransom payment.

The Sirius Star held 2 million barrels of oil valued at about $100 million and was released last January.

The seizure of the tanker will have a minimal effect on global oil markets, said Ben Cahill, the petroleum risk manager at global oil consultancy PFC energy, although American refineries in the Gulf of Mexico region waiting for the ship's crude might experience some disruption.

Cahill said the longer term concern for the oil industry was that Yemen would collapse like its neighbor Somalia, and create an "arc of instability" that would block the mouth of the Gulf of Aden. Somalia's lawless 1,880-mile coastline already provides a perfect pirate haven.

The impoverished Horn of Africa nation has not had a functioning government for a generation and the weak U.N.-backed administration is too busy fighting the Islamist insurgency to arrest pirates. Across the Gulf of Aden, tensions between north and south Yemen continue to rise and Islamic militancy is increasing.

The situation could degenerate into a choke point for global shipping," in five to ten years, Cahill said, driving up the costs.

Pirates now hold about a dozen vessels hostage and more than 200 crew members.