Somali pirates pulled off their most audacious hijack to date yesterday, venturing outside their usual waters to seize a Saudi Arabian supertanker laden with two million barrels of crude, in an attack that sent oil prices higher.

The vessel – three times the size of a US aircraft carrier – was taken 450 miles off the coast of Kenya and well beyond the Gulf of Aden where the pirates have been stalking international shipping. Its seizure marked a "fundamental change" in tactics, the US Navy said.

Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, suggested military intervention would be complicated by hostages and ransom demands. “I’m stunned by the range of it,” said Admiral Mullen, adding that this was the furthest from the African coast that pirates had struck.

“Once they get to a point where they can board, it becomes very difficult to get them off because, clearly, now they hold hostages.”

There were no reports of damage to the ship or injury to the 25 crew aboard, which includes two Britons. But the capture of $100m worth of oil represents a dramatic show of strength from the Somali pirates who traditionally operate from speedboats, using automatic weapons and rocket launchers. News of the hijacking pushed up the price of crude by 2 per cent yesterday afternoon.

The Sirius Star, owned by Saudi oil giant Aramco, was last night headed for the Somali coast, said Lieutenant Nathan Christensen, from the US Fifth Fleet, based in Bahrain. "This is the furthest and largest target the pirates have taken," he said. "The information we have is that it is headed for an anchorage point off the Somali port of Eyl."

The coastal town in Somalia's breakaway region of Puntland is at the epicentre of an explosion of piracy: attacks have increased dramatically, with more than 60 vessels hijacked so far this year. They include the MS Vaina, a Ukrainian tanker with a shipment of arms and munitions, including 33 Soviet-era tanks, which is still being held. The entire economy of Eyl now runs on piracy, locals say, with the armed men who are hijacking ships enjoying hero status in the port city.

The Sirius Star, classed as a "very large crude carrier", was fully laden, with the two million barrels of oil, her owners said, meaning that the equivalent of a quarter of Saudia Arabia's daily output was in pirate hands.

There was still some confusion over the whereabouts of the vessel last night. "It has not entered Puntland's waters so far," Abdulqadir Muse Yusuf, the assistant Minister for Fisheries, told Reuters. But sources in Eyl said arrival of the supertanker was being eagerly awaited. The town is already experiencing an economic boom, with new houses, cars and roads appearing in a country otherwise shattered by decades of conflict.

One of the self-proclaimed pirate leaders, Bile Wadani, had predicted by phone from an unidentified vessel to The Independent last week that new attacks would be launched further off the coast in defiance of increased foreign patrols in the Gulf of Aden. He said that ships as far afield as Yemen would be taken.

The seizure of an oil tanker will add dramatically to the pressure for international action to protect one of the most valuable shipping lanes in the world. Its capture raises the spectre of a possible "environmental disaster" in east Africa, a prospect that the British think-tank Chatham House raised in a report last month. "As pirates become bolder and use ever more powerful weaponry, a tanker could be set on fire, sunk or forced ashore, any of which could result in an environmental catastrophe that would devastate marine and bird life for years to come," it said. "The pirates' aim is to extort ransom payments and to date that has been their main focus; however, the possibility that they could destroy shipping is very real."

Authorities in Puntland say they can do little to stop the pirates, and blame ship-owners for causing the crisis by paying ransoms, estimated to be more than $30m (£20m) this year alone.

British marines last week killed three people identified as pirates in a skirmish after an attack on a Danish vessel. Japan, Denmark, France and Russia have naval patrols in the region, in addition to the US and Britain. In April, French forces attacked pirates en route to Eyl after collecting a ransom for the release of a luxury yacht and its crew. Six of those captured will stand trial in France.

Insurers have increased operators' premiums, forcing some of them to consider longer, alternative shipping routes, which could eventually increase the cost of everyday items.

The pirates themselves claim that they have been forced into attacking international shipping after the destruction of their coastal fisheries over the past decade. Without an effective government for nearly two decades, Somalia has descended into a violent anarchy, a vacuum exploited by commercial fishing operations, many from Europe, to cash in on tuna and unspoilt fishing grounds.

On land, Somalia is already in effect divided into three parts, with the breakaway Somaliland and Puntland taking no orders from the capital, Mogadishu. The leadership of the discredited transitional government in rump Somalia has admitted it is on the point of collapse. Rival Islamic insurgencies control the bulk of its territory and the transitional government is kept in business by an occupying force from Ethiopia. An additional force of United Nations peacekeepers is hemmed into a small area of the capital and no one group holds effective sway over the country.