In one of their boldest hijacks yet, Somali pirates have seized a massive, Saudi-owned oil supertanker loaded with crude oil and carrying 25 crew members off the Kenyan coast, the U.S. Navy said Monday.

The hijacking was the latest in a surge in attacks this year by ransom-hungry Somali pirates and highlighted the vulnerability of even very large ships moving through the area. Attacks off the Somali coast have increased more than 75 percent this year.

After the brazen hijacking, the pirates took the ship to a Somali port that has become a haven for bandits and the ships they have seized, a Navy spokesman told the Associated Press.

The tanker, owned by Saudi oil company Aramco and operated by Vela International, is 1,080 feet, about the length of an aircraft carrier, making it one of the largest ships to sail the seas. It can carry about 2 million barrels of oil.

Lt. Nathan Christensen, a spokesman for the U.S. Navy's 5th Fleet, said the Sirius Star was carrying crude at the time of Saturday's hijacking, but he did know how much. He also had no details about where the ship was sailing from and where it was headed.

"The Sirius Star ... was seized by a group of armed men approximately 420 nautical miles off the coast of Somalia on Sunday," a Vela International spokesman said. "All 25 crew members on board are reported to be safe and the vessel is fully laden with crude oil. Vela Response Teams have been mobilized and are working to ensure the safe release of the crew members and the vessel."

While Saudi-owned television station Al Arabiya reported that the crew was released by the hijackers, both the U.S. Navy and Aramco said they had not received such information.

The ship was sailing under a Liberian flag and its 25-member crew includes citizens of Croatia, Britain, the Philippines, Poland and Saudi Arabia.

In a news release sent out on Monday from the 5th Fleet's Middle East headquarters in Bahrain, the Navy said the large crude tanker Sirius Star was attacked more than 450 nautical miles southeast of Mombasa, Kenya, an area far south of the zone patrolled by international warships.

It was the farthest Somali pirates have traveled so far to hijack a ship, Christensen said.

By expanding their ability to attack so far out at sea, Somali pirates are "certainly a threat to many more vessels," Christensen said.

A British Foreign Office spokesman said there were at least two British nationals aboard the MV Sirius Star, but said he could offer no further details on the ship or what had happened to it.

The Sirius Star was built in South Korea's Daewoo shipping yards and commissioned in March. Classed as a Very Large Crude Carrier, the ship is 318,000 dead weight tons.

An operator with Aramco said there was no one available at the company to comment after business hours.

As pirates have become better armed and equipped, they have sailed farther out to sea in search of bigger targets, including oil tankers, among the 20,000 tankers, freighters and merchant vessels transiting the Gulf of Aden each year.

Somali pirates are trained fighters, often dressed in military fatigues, using speedboats equipped with satellite phones and GPS equipment. They are typically armed with automatic weapons, anti-tank rockets launchers and various types of grenades.

Raja Kiwan, a Dubai-based analyst with PFC Energy, said the hijacking raises "some serious questions" about what is needed to secure such ships when they are on the open seas.

"It's not easy to take over a ship" as massive as an oil tanker, particularly VLCC's that can transport about 2 million barrels of crude, he said, adding that such vessels typically have an armed security contingent on board.

Pirates have gone after oil tankers before.

In October, a Spanish military patrol plane thwarted pirates trying to hijack an oil tanker by buzzing them three times and dropping smoke canisters.

On April 21, pirates fired rocket-propelled grenades at a Japanese oil tanker, leaving a hole that allowed several hundred gallons of fuel to leak out, raising fears for the environment.

In September, three pirates in a speed boat fired machine guns at an Iranian crude oil carrier, though the ship escaped after a 30-minute chase.

Warships from the more than a dozen nations as well as NATO forces have focused their anti-piracy patrols in the Gulf of Aden, increasing their military presence in recent months.

But Saturday's hijacking occurred much farther south, highlighting weaknesses in the international response to the problem.

Graeme Gibbon Brooks, the managing director of British company Dryad Maritime Intelligence Service Ltd, said the increased international presence trying to prevent attacks is simply not enough.

"The coalition has suppressed a number of attacks ... but there will never be enough warships. The whole area is 2.5 million square miles ... the coalition have to act preemptively and be one step ahead of the pirates. The difficulty here is that the ship was beyond the area where the coalition were currently acting."

He did not know whether the Saudi ship had weapons or a security team onboard, but said their location — 200 kilometers off the coast — may have given the crew a false sense of security.

Brooks said the tanker likely had been targeted by a group of pirates distinct from the attackers in Somalia's Puntland region in the north, a notorious piracy hotspot.

The pirates in southern Somalia have not carried out any attacks this year, he said, probably because warships escorting food shipments from Mombasa to Mogadishu had been a deterrent.

"But now they see Puntland pirates appear to be operating impervious to the coalition. Perhaps they've drawn the same conclusion, that they can continue to carry out attacks," he said.

FOX News' Greg Palkot and The Associated Press contributed to this report.