It added there was no specific or imminent threat and said officials didn't know whether al-Qaida had continued the plotting since last year.
In a confidential warning obtained by The Associated Press, the FBI and the Homeland Security Department said that al-Qaida sought information on the size and construction of oil tankers, and determined that blowing them up from the inside would be easiest due to the strength of their hulls. Al-Qaida recommended test runs of the plot.
"We are not aware of indications of any specific or imminent terrorist attack plotting against the oil and natural gas sector overseas or in the United States," DHS spokesman Matthew Chandler said in a statement Friday. "However, in 2010 there was continuing interest by members of al-Qaida in targeting oil tankers and commercial oil infrastructure at sea."
The government warning went to federal, state and local law enforcement and companies in the oil and gas industries. The Homeland Security Department said it was not raising the nation's terror alert level.
The government encouraged companies to continue random screening, warn employees about possible threats and establish procedures for reporting suspicious activity.
The threat to oil tankers was especially notable because it suggested that al-Qaida was adopting the practices of Somali pirates, who have had remarkable success in recent years using small boats to race alongside and board large commercial ships off the East African coast. They hold the cargo and crew for economic ransom.
Al-Qaida successfully attacked the USS Cole in October 2000, killing 17 sailors, using a small boat loaded with explosives when the warship was docked in the port of Aden in Yemen, which is just across the Gulf of Aden from Somalia.
The government said al-Qaida recognized that the period from mid-spring to late summer would represent the best weather to approach oil tankers. Al-Qaida determined that sinking tankers from the outside would require too many explosive charges, and instead focused on detonating the fuel storage areas aboard tankers.
In 2007 the Japanese tanker the Golden Nori was hijacked carrying 40,000 tons of benzene. Initially, American intelligence agents worried that terrorists from Somalia's Islamic extremist insurgency could be involved and might try to crash the boat into an offshore oil platform or use it as a gigantic bomb in a Middle Eastern port. When the Japanese vessel was towed back into Somali waters and ransom demanded, the coalition was relieved to realize it was a pirate attack.
Associated Press writer Katharine Houreld contributed to this story from Nairobi.