Hijacked ... The giant tanker Sirius Star.
Photo: AP

The top US military officer says he is "stunned" by the reach of Somali pirates who seized a Saudi supertanker - the size of a US aircraft carrier - 730 kilometres off the coast of Africa.

Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff, called piracy a growing problem that needed to be addressed.

But he said there were limits to what the world's navies could do once a ship had been captured because national governments often preferred to pay pirates ransom.

"I'm stunned by the range of it, less so than I am the size," Admiral Mullen said of the seizure of the Sirius Star on Sunday by armed men.

The huge, oil-laden prize was about 730 kilometres east of Kenya when it was boarded, he said.

That is the furthest out at sea that a ship has been seized in the latest surge of piracies, Admiral Mullen said.