At least three other ships have been taken since the Sirius Star was captured last Saturday, and more than one dozen vessels with about 280 sailors on board are being held in Somali waters.
More than 120 attacks have been reported off the coast of Somalia this year.
Al Jazeera's Mohammed Adow, reporting from the Kenyan port of Mombasa, said that more Somalis were becoming involved in piracy every day because of the money that can be made from the huge ransoms paid by ship owners.
"They are being joined by former fishermen, former officials of the Somali navy and even students," he said.
The International Association of Independent Tanker Owners (Intertanko) has called for international action to tackle the situation and said many operators were considering sending their vessels around the tip of South Africa to avoid pirates.
"Piracy, as well as the recent terrorist attacks against international targets, are only symptoms of the fundamental problem which is the state of anarchy in Somalia"
UN Assistant Secretary-General for Peacekeeping
"We need immediate action from governments to protect these vital trade lanes - robust action in the form of greater naval and military support with a clear mandate to engage, to arrest pirates and to bring them to trial," Intertanko said.
In a statement on Thursday, Denmark's AP Moeller-Maersk shipping company said vessels that were too slow or had decks low enough for the pirates to scramble aboard would "seek alternative routing" around the Cape of Good Hope and Madagascar.
Norwegian shipping company Odfjell said on Monday that it would also use the longer, more expensive route.
Efthimios Mitropoulos, the head of the International Maritime Organisation, spoke of "a series of negative repercussions" if ships had to reroute.
He said going around the Cape added about 12 days to a typical Gulf-to-Europe voyage, delaying oil supplies, and potentially raising freight rates by 25 to 30 per cent.
Mitropoulos said the UN Security Council needed to strengthen the mandate of anti-piracy forces with "clear rules of engagement" and to make states bring to justice pirates they captured.Anti-piracy force
Nato sent four warships into the Gulf of Aden last month to prevent piracy and escort aid vessels, while a European Union anti-piracy operation off the coast of Somalia is set to begin on December 8.
Several other nations, including Russia and India, also have navy vessels working to protect shipping in the area.
The Security Council on Thursday voted unanimously to impose sanctions on pirates, arms smugglers and other people involved in fuelling the instability in Somalia, which has been without a functioning central government since 1991.
"Piracy, as well as the recent terrorist attacks against international targets, are only symptoms of the fundamental problem which is the state of anarchy in Somalia," Raisuddin Zenenga, UN assistant secretary-general for peacekeeping, said in New York.