Somali pirates seized a tanker carrying more than $20 million of crude oil from Saudi Arabia to the United States in the increasingly dangerous waters off East Africa, an official said Monday, an attack that could pose a huge environmental or security threat.

The Greece-flagged Maran Centaurus was hijacked Sunday about 800 miles (1,300 kilometers) off the coast of Somalia, said Cmdr. John Harbour, a spokesman for the EU Naval Force. Harbour said it originated from Jeddah, Saudi Arabia and was destined for the United States. The ship has 28 crew members on board, he said.

The shipping intelligence company Lloyd's List said the Maran Centaurus is a "very large crude carrier, with a capacity of over 300,000 tons."

Stavros Hadzigrigoris from the ship's owners, Maran Tankers Management, said the tanker was carrying around 275,000 metric tons of crude. At an average price of around $75 a barrel, the cargo is worth more than $20 million. Hadzigrigoris declined to say who owned the oil.

Pirates have increased attacks on vessels off East Africa for the millions in ransom that can be had. Though pirates have successfully hijacked dozens of vessels the last several years, Sunday's attack appears to be only the second ever on an oil tanker.

The hijacking of a tanker increases worries that the vessel could crash, be run aground or be involved in a firefight, said Roger Middleton, a piracy expert at London-based think tank Chatham House.

Pirates typically use guns and rocket-propelled grenades in their attacks, and some vessels now carry private security guards, but Middleton said oil tankers do not.

"You're sitting on a huge ship filled with flammable liquid. You don't want somebody with a gun on top of that," Middleton said. "Financially it's a very costly exercise because the value of oil is so volatile. If it is held for a long time and the price of oil drops, they could lost millions of dollars."

In November 2008, pirates hijacked the Saudi supertanker Sirius Star, which held 2 million barrels of oil valued at about $100 million. The tanker was released last January for a reported $3 million ransom after a two-month drama that helped galvanize international efforts to fight piracy off Africa's coast.

Somali pirates are a separate group of criminals from the al-Qaida-affiliated Islamic militants who control large areas of southern Somalia, but anytime pirates hold such valuable and explosive cargo it raises international concerns.

In late 2007, pirates hijacked a chemical tanker carrying up to 10,000 tons of highly explosive benzene. Initially, American intelligence agents worried terrorists from Somalia's Islamic extremist insurgency could be involved, and might try to crash the boat into an offshore oil platform or use it as a gigantic bomb.

When the Japanese vessel was towed back into Somali waters and ransom demanded, the coalition was relieved to realize it was just another pirate attack.

Somalia's lawless 1,880-mile (3,000-kilometer) coastline provides a perfect haven for pirates to prey on ships heading for the Gulf of Aden, one of the world's busiest shipping routes. 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.

Pirates now hold about a dozen vessels hostage and more than 200 crew members. The Maran Centaurus had 28 crew aboard — 16 Filipinos, nine Greeks, two Ukrainians and one Romanian, Harbour said.

Middleton said pirate demands and negotiations are becoming more complex.

"They still want the money but they have also asked for the release of imprisoned comrades," he said. "That demand is an extra bargaining tool they can use to add extra layers to their negotiating position."

Piracy has increased despite an increased presence by international navies patrolling the Indian Ocean and Gulf of Aden. The U.S. this fall began flying sophisticated drones over East African waters as part of the fight against piracy.

Why have the world's most advanced navies failed to end piracy in the seas around Somalia?

Cdr Bindts say the pirates adapt quickly to his taskforce's tactics

The Dutch frigate Evertsen is a reassuring sight for the civilian ships dotted around the horizon as she ploughs steadily through the calm, glittering waters of the Gulf of Aden.

But all the bristling firepower of the EU's anti-piracy task force has not been enough to remove the threat of piracy from the seas around Somalia.

Why has it been so difficult for the world's most advanced navies to defeat pirates who are armed with just Kalashnikovs and rocket- propelled grenades?

It is true there has not been a successful hijacking since July in the Gulf of Aden, the corridor between Yemen and Somalia which leads to the Suez Canal. That is of enormous importance, since 20% of the world's shipping travels this way.

'Tiny chance'

But the pirates have not been defeated. They have just moved south into the Indian Ocean, continuing to plague the waters known as the Somali Basin. This is where the British couple, Paul and Rachel Chandler, were seized from their yacht.

The task force and other navies patrol an area the size of Western Europe

The first problem for the European force is one of simple geography.

Along with other navies concerned about the piracy problem, it has to patrol an area the size of western Europe. They could be several days away from a vessel when it is boarded by pirates.

So there is only a tiny chance of catching the pirates in the act of trying to board a civilian vessel - and even then, the warships are limited in what they can do. Often they cannot use the immense firepower at their disposal.

"This is not so much an enemy, that would sound like a war - and we're doing legal work with military means," says Cdr Pieter Bindt, commander of the EU's anti-piracy taskforce.

"They [the pirates] are very adaptive; they react to what we do and they have a very large area where they can start from: the Somali coast, which is thousands of miles long."

'Why not just blow them out of the water?' I asked.

"In the Western world we like to have due process in legal issues," he said. "It would be the same as if somebody in London looking like a burglar would be shot on sight, we just don't that."

'Knowing the law'

In the Evertsen's operations room, a red square on a nautical map shows the progress of a PAG, or Pirate Action Group.

The pirates are currently holding 10 merchant ships

This is usually two small skiffs, or speed boats just a few metres long, and a mother skiff, slightly bigger, carrying food, fuel and ammunition.

A Greek frigate has been sent to intercept this particular PAG, but after boarding the tiny skiffs, the suspected pirates are questioned and released.

The pirates know the law. When they see a naval frigate coming, they dump their weapons, boarding ladder, and even satellite telephones over the side. This is what has happened with the pirates being tracked in the operations room.

Everyone fully expects them simply to return to shore to re-equip themselves and a few days later set out to sea again, hunting for vulnerable ships. The ransoms, often several million dollars, are enough to comfortably pay for new equipment.

'Like catching junkies'

This is all very frustrating for the task force.

The Evertsen's captain, Cdr Cees Vooijs, told me about a typical incident in which a skiff had been seen approaching a merchant vessel to attempt a boarding. It had then been intercepted after it fled the scene.

"When we boarded the skiff we found nothing - nothing of piracy things - and their story was that they were fishermen but we found nothing that explained that they are fishermen, no fish, no smell of fish, no fishing gear.

"For us it was a very clear case that they did the attack... that was not enough for the public prosecutor in the Netherlands to say there should be a trial and that was a very bad moment."

The task force may even have encountered the same groups of pirates several times, having released them on previous occasions.

"It is just like the policemen in Amsterdam catching junkies who steal bicycles," said one of the Dutch officers on board. "They kick them out after arresting them and then see them back in the police station the following morning."

There have been a number of prosecutions but there has yet to be a single conviction.

So, with huge sums at stake, the pirates seem to be as busy as ever and the waters off Somalia remain dangerous for merchant vessels.

Armed escorts

Currently, the pirates are holding 10 merchant ships. They can be seen lying at anchor off the Somali coast; their crews - totalling some 236 people - are hostages.

The EU force could overwhelm the hijackers. But the risk to the hostages is too great. Once in possession of a ship, the chances are the pirates will collect their ransom.

"If the pirates are already on board, then there is not much that we can do - if you value the lives of the hostages," said Cdr Bindt.

All of the foreign naval firepower now deployed here is a potent deterrent but the pirates are remarkably persistent.

No surprise then that a number of merchant navies, including the French and the Spanish, are starting to take armed personnel with them on board.

The sailors here like to say that the problem is as much on land as at sea. In other words, there will be pirates as long as there is chaos and instability in Somalia itself.