WASHINGTON — The terror group al-Qaida last summer considered hijacking and detonating oil tankers in non-Muslim seas to provoke an “extreme economic crisis” in the West, according to documents seized from Osama bin Laden’s compound in Pakistan, the U.S. government said.

It said there was no specific or imminent threat and that officials don’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.”

Another U.S. official, who was briefed on the bin Laden documents, described the plot as aspirational and not fully formed. The official spoke on condition of anonymity to discuss intelligence matters.

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.

There was no immediate effect on oil markets. West Texas Intermediate crude oil, the benchmark for oil prices in the U.S., declined more than 2 percent early in the day.

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 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 attacked the USS Cole in October 2000, using a small boat loaded with explosives while the warship was docked in the port of Aden in Yemen, just across the Gulf of Aden from Somalia. Seventeen sailors were killed.

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. Al-Qaida hoped the destruction of a tanker — presumably along with cleanup and salvage costs and market effects — would result in “extreme economic crisis” in the West.

But experts said al-Qaida probably overestimated what the loss of a single oil tanker might be on the industry. Concerns about the safety of individual oil tankers from pirate activity or terrorism hasn’t really affected oil prices, said Jim Ritterbusch, president of Ritterbusch and Associates, who has been trading oil contracts since the futures market opened on the Nymex in 1983. A tanker holds about 2 million barrels. That’s enough to supply world demand for only about a half hour.

“These are isolated incidents, and the actual quantity of any one tanker isn’t that big of a deal,” Ritterbusch said.

In 2007 the Japanese tanker the Golden Nori was hijacked carrying 40,000 tons of the highly explosive chemical 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.

A Western diplomat said that the tanker had been closely monitored throughout the time it was held and that naval forces would have fired on the ship and blown it up rather than allow it to be sailed into a port. He asked for anonymity because he was not authorized to speak to the press.

Oil markets were skittish shortly after Sept. 11, 2001, and that elevated prices slightly. But after a decade of relative safety, oil traders have started to cut back on the terrorist fear premium, Ritterbusch said.

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