NAIROBI, Kenya (CNN) -- Pirates who hijacked a crude oil tanker off the coast of Kenya are approaching a Somali port, the U.S. Navy says.
The Sirius Star -- a crude "super tanker" flagged in Liberia and owned by the Saudi Arabian-based Saudi Aramco company -- was attacked on Saturday more than 450 nautical miles southeast of Mombasa, Kenya.
The crew of 25, including British, Croatian, Polish, Filipino and Saudi nationals, are reported to be safe.
U.S. Navy Fifth Fleet Cmdr. Jane Campbell said the super tanker weighs more than 300,000 metric tons and "is more than three times the size of a U.S. Navy aircraft carrier."
Oil industry insiders say a tanker of this size can carry up to 2 million barrels of oil, and the ship's operator, Dubai-based Vela International Marine Ltd, says it is fully laden.
A U.S. Navy spokesman said the tanker is approaching Eyl, Somalia, on the Indian Ocean coast. It is routine procedure for pirates to take hijacked ships to shore, where they will keep them while they discuss negotiations.
A multinational naval force including vessels from the U.S., the UK and Russia has been patrolling the Indian Ocean waters seas near the Gulf of Aden, which connects the Red Sea and the Arabian Sea, following a sharp increase in pirate attacks in the region.
"It was attacked more than 450 nautical offshore of Mombasa. This means that the pirates are now operating in an area of over 1.1 million square miles. This is a measure of the determination of the pirates and ... a measure of how lucrative piracy could become," Campbell said.
Campbell said the Navy does not expect to dispatch a vessel to aide the super tanker because it does not have dangerous weapons aboard like the MV Faina, a Ukrainian ship loaded with arms that was seized by pirates on September 25.
The UK Foreign Office confirmed two Britons were aboard and said it was seeking more information about the incident.
South Korean officials said on Sunday that armed gunmen hijacked a Japanese freighter and its 23-member crew off the coast of Somalia. The hijacking came as the Korean government was considering sending a warship to join those of other countries to combat piracy in the area.
A Russian patrol ship also thwarted an attack on a Saudi vessel.
Eleven vessels are currently being held by pirates hoping to secure ransoms for their release, according to The Associated Press. They include the MV Faina, which was hijacked along with 20 crew and a cargo of weapons and T-72 tanks.
Ninety percent of ships transiting the perilous seas are using a guarded corridor and there have been no hijackings inside the zone since it was set up on August 22, Danish Commodore Per Bigum Christensen told AP last week.
Around 20,000 oil tankers, freighters and merchant vessels pass along the crucial shipping route each year.
"Our presence in the region is helping deter and disrupt criminal attacks off the Somali coast, but the situation with the Sirius Star clearly indicates the pirates' ability to adapt their tactics and methods of attack" said U.S. Vice Admiral Bill Gortney, commander of the Combined Maritime Forces.
"Piracy is an international crime that threatens global commerce. Shipping companies have to understand that naval forces can not be everywhere. Self protection measures are the best way to protect their vessels, their crews, and their cargo."
CNN's David McKenzie contributed to this report