Somali pirates who hijacked an oil tanker with 25 crew, including two Britons, on board have opened negotiations. Vela International Marine Ltd, the Dubai-based marine company that operates the Saudi-owned Sirius Star, said it was working to secure the release of the supertanker and her crew. A spokesman for the company said: "All 25 crew members are reported to be safe and the vessel is fully laden. "A Vela response team has been established and is working to ensure the safe release of the crew members and the vessel." A spokesman for the Foreign Office confirmed that two of those on board are British but could not give any details of their role on the ship. He said: "We are seeking more information on the incident." The hijack, which was the first successful attack so far out at sea, raises fears that international patrols nearer the coast and in the Gulf of Aden will not be enough to protect vital trade routes as pirate gangs become ever more audacious. The vessel is the second of six Very Large Carriers manufactured by Dubai-based Vela International and set sail on its maiden voyage in March this year. A spokesman for the Royal Navy said he could not say if British servicemen were involved in any attempts to rescue the vessel. The Sirius Star was seized 520 miles southeast of Mombasa, Kenya, in a shipping zone where Somali raiders strike almost daily. The ship is carrying as much as two million barrels of oil - more than a quarter of daily Saudi Arabia's daily exports and reports of the hijacking sparked rises in global crude oil prizes. She had been heading for the United States via the Cape of Good Hope at the southern tip of Africa, skirting the continent instead of heading through the Gulf of Aden and the Suez Canal. Lieutenant Nathan Christensen, a spokesman for the US Fifth Fleet, said: "This is unprecedented. It's the largest ship that we've seen pirated. It's three times the size of an aircraft carrier." There were no reports of damage to the ship, Lt Christensen said. He declined to say if the US Navy was considering taking action to rescue the tanker, saying: "We are evaluating the situation." He added: "The latest information we have is that they are taking the vessel to a Somali port. "We don't know the condition of the crew on board or the nature of the pirates' demands. In cases like this what we typically see is a demand for money from the ship owners but we haven't had that yet. "We don't know exactly where they are taking it but we know the town of Eyl is a pirate stronghold." Eyl is in the northern Puntland region of Somalia and has become notorious for pirate activity over the past months. Dozens of ships are thought to be being held captive there. Chaos in Somalia, where Islamist forces are fighting a Western-backed government, have spawned a wave of piracy. Shipowners have paid out millions of dollars in ransoms. Well over 60 vessels have been hijacked this year, driving up shipping insurance premiums and pushing some vessels to take longer routes between Asia and Europe than pass through the Suez Canal, increasing the cost of traded goods.