But with naval forces unwilling to intervene, shipowners in past piracy cases have ended up paying ransoms for their ships, cargos and crew.
The latest ship seized was a bulk cargo carrier flying a Hong Kong flag and operated by the Islamic Republic of Iran Shipping Lines. U.S. Navy Commander Jane Campbell of the Bahrain-based 5th Fleet said the status of the crew and cargo was not known.
The International Maritime Bureau on Sunday reported five hijackings since Nov. 7, before the hijackings of the Saudi and Iranian ships were announced.
The U.S. and other naval forces decided against intervention in the seizure of the supertanker. NATO said it would not divert any of its three warships from the Gulf of Aden and the U.S. Navy's 5th Fleet also said it did not expect to send ships to try to intercept the Saudi supertanker, the MV Sirius Star. The tanker was seized over the weekend about 450 nautical miles off the Kenyan coast.
Never before have Somali pirates seized such a giant ship so far out to sea - and never a vessel so large. The captors of the Sirius Star anchored the ship, with a full load of 2 million barrels of oil and 25 crew members, close to a main pirate den on the Somali coast, Harardhere.
"As usual, I woke up at 3 a.m. and headed for the sea to fish, but I saw a very, very large ship anchored less than three miles off the shore," said Abdinur Haji, a fisherman in Harardhere.
"I have been fishing here for three decades, but I have never seen a ship as big as this one," he told The Associated Press in a telephone interview. "There are dozens of spectators on shore trying to catch a glimpse of the large ship."
He said two small boats floated out to the ship and 18 men - presumably other pirates - climbed aboard with a rope ladder. Spectators watched as a small boat carried food and qat, a narcotic leaf popular in Somalia, to the supertanker.
Saudi Foreign Minister Prince Saud al-Faisal called the hijacking "an outrageous act" and said "piracy, like terrorism, is a disease which is against everybody, and everybody must address it together."
Speaking during a visit to Athens on Tuesday, he said Saudi Arabia would join an international initiative against piracy in the Red Sea area, where more than 80 pirate attacks have taken place this year.
He did not elaborate on what steps the kingdom would take to better protect its vital oil tankers. Saudi Arabia's French-equipped navy has 18,000-20,000 personnel, but has never taken part in any high-seas fighting.
Abdullkadir Musa, the deputy sea port minister in northern Somalia's breakaway Puntland region, said if the ship tries to anchor anywhere near Eyl - where the U.S. earlier said it was heading - then his forces will try to rescue it.
Forces from Puntland have sometimes confronted pirates, though Somalia's weak central government, which is fighting Islamic insurgents, has been unable to mount a response to increasing piracy.
Puntland forces, their guns blazing, freed a Panama-flagged cargo ship from pirates on Oct. 14.
The Dubai-based owner of the Saudi tanker, Vela International Marine Ltd., said the oil tanker's 25 crew members "are believed to be safe." The statement made no mention of a ransom or contacts with the bandits.
The Sirius Star's cargo is worth about $100 million at current prices, but the pirates have no known way to unload it from the tanker.
In Vienna, Ehsan Ul-Haq, chief analyst at JBC Energy, said the seizure was not affecting oil prices, since traders were focused instead on "the overall economy."
The U.S. Navy is still surrounding a Ukrainian ship loaded with tanks and other weaponry that was seized by pirates Sept. 25 off the Somali coast.