Negotiations for the crew's release could be costly and protracted, says BBC security correspondent Frank Gardner.
The pirates are a sophisticated group with contacts in Dubai and neighbouring countries, says the BBC Somali Service's Yusuf Garaad.
Much of their ransom money from previous hijackings has been used to buy new boats and weapons as well as develop a network across the Horn of Africa, he adds.
There have been at least 90 attacks by Somali pirates this year and more than 200 crew are still being held for ransom along the Somali coast.
A South Korean-built vessel, that is about the length of a US aircraft carrier, the Sirius Star was heading for the US via the southern tip of Africa when it was hijacked in what Saudi Foreign Minister Prince Saud Al-Faisal condemned as "an outrageous act".
The US Navy said the seizure was an "unprecedented" attack and confirmed that the tanker had anchored off the Somali coast near the town of Harardhere.
Vela International said the crew consisted of two British, two Polish, one Croatian, one Saudi and 19 Philippine nationals. The captain is a Pole, Poland's foreign ministry confirmed.
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