Such vessels can command higher ransoms because of the value of the crude on board. Owners of the oil may want to resolve hostage situations quickly, particularly if oil prices are dropping, a situation that can cost owners millions of dollars more than the pirate ransom will.
Still, ransom prices are on the rise. One last year reached $9.5 million, and the increasing prizes have provided even more incentive for pirates to launch attacks despite stepped-up patrols by an international flotilla of warships.
Pirates hold 29 ships and roughly 660 hostages.
The Irene SL was sailing 200 nautical miles (360 kilometers) east of Oman with a cargo of 266,000 tons of crude oil and a crew of seven Greeks, 17 Filipinos and one Georgian when it was attacked on Wednesday, Greece's Merchant Marine Ministry said. The Associated Press estimated the value of the oil at more than $150 million, based on the amount being carried and a price of $87 a barrel.
The tanker was sailing from the Persian Gulf to the Gulf of Mexico. The ministry said authorities had lost contact with the ship since the attack.
The Piraeus-based shipping company First Navigation Special Maritime Enterprises confirmed its ship had been attacked but had no further comment.
The Irene SL was the second oil tanker to be attacked in that region in two days. On Tuesday, Somali pirates firing small arms and rocket-propelled grenades hijacked an Italian-flagged oil tanker in the Indian Ocean that had been heading from Sudan to Malaysia.
Meanwhile, South Korea's foreign ministry said pirates on Wednesday released a South Korean fishing vessel that was captured nearly four months ago while fishing for crabs. The ministry said 39 Kenyans, two South Koreans and two Chinese sailors were aboard the Keummi 305. The ship was last reported sailing away from Somalia toward international waters.
A vessel from the EU Naval Force was sailing toward the ship to secure the safety of sailors at the request of a South Korean navy vessel, the ministry said. The 241-ton Kenya-registered trawler was attacked Oct. 9 in the waters off Kenya's Lamu Island.
No information about a ransom was reported, but pirates only release ships after being paid.
British Royal Navy Lt. Cmdr. Susie Thomson, a spokeswoman for the multinational Task Force 152 based in Bahrain, said that pirates have been waging attacks increasingly deeper into the Gulf of Oman and the Indian Ocean.
Pirates have launched attacks more than 1,000 miles (1,600 kilometers) east of Somalia.
Associated Press writers Kim Kwang-tae in Seoul, South Korea, and Elena Becatoros in Athens, Greece contributed to this report.