Somali pirates have "stunned" the United States' top military leader by seizing a supertanker three times as big as an aircraft carrier hundreds of nautical miles off the Kenya coast.

Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said on Monday that he was surprised both by the size of the vessel the pirates had hijacked and by its distance from the shore.

"What strikes me about this particular supertanker is how far away from Africa it was. As I understand it, it's about 450 miles south east of Kenya," he told a Pentagon briefing in Washington, DC. "That's the longest distance I've seen for any of these incidents."

The BBC quoted a naval spokesman as saying the tanker, the Sirius Star, was headed for the Somali coast, where it was "nearing an anchorage point" at Eyl, a port in Puntland which is popular with Somali pirates.

The Sirius Star is the biggest ship to have been seized by Somali pirates so far. It has 25 crew members aboard and was heading to the U.S. via the Cape of Good Hope.

The Combined Maritime Forces (CMF) of the U.S.-led military coalition which patrols the Red Sea, the Arabian Gulf and the Indian Ocean have recorded a reduction in successful piracy attacks from 53 percent in August to 31 percent in October.

However, Mullen noted that despite the drop in the proportion of attacks that succeeded, "we've seen an extraordinary rise in the overall numbers."

The Liberian-flagged vessel is operated by the Dubai – based company, Vela International. Al Jazeera reported that it was carrying almost U.S. $100 million worth of oil. In the past, such hijackings have usually been followed by ransom demands issued by the pirates to ship owners.