Islamist militias in Libya took control of nearly a dozen commercial jetliners last month, and western intelligence agencies recently issued a warning that the jets could be used in terrorist attacks across North Africa.

Intelligence reports of the stolen jetliners were distributed within the U.S. government over the past two weeks and included a warning that one or more of the aircraft could be used in an attack later this month on the date marking the anniversary of the Sept. 11, 2001, terrorist attacks against New York and Washington, said U.S. officials familiar with the reports.

"There are a number of commercial airliners in Libya that are missing," said one official. "We found out on September 11 what can happen with hijacked planes."

The official said the aircraft are a serious counterterrorism concern because reports of terrorist control over the Libyan airliners come three weeks before the 13th anniversary of 9/11 attacks and the second anniversary of the Libyan terrorist attack on the U.S. diplomatic compound in Benghazi.

Four Americans, including Ambassador Christopher Stevens, were killed in the Benghazi attack, which the Obama administration initially said was the result of a spontaneous demonstration against an anti-Muslim video.

A senior State Department counterterrorism official declined to comment on reports of the stolen jetliners.

A second State department official sought to downplay the reports. "We can’t confirm that," he said.

Meanwhile, officials said Egyptian military forces appear to be preparing to intervene in Libya to prevent the country from becoming a failed state run by terrorists, many with ties to al Qaeda.

Libya remains an oil-rich state and if the country is taken over completely by Islamist extremists, U.S. counterterrorism officials believe it will become another terrorist safe haven in the region.

The officials said U.S. intelligence agencies have not confirmed the aircraft theft following the takeover of Tripoli International Airport in late August, and are attempting to locate all aircraft owned by two Libyan state-owned airline companies, as security in the country continued to deteriorate amid fighting between Islamists and anti-Islamist militias.

Video surfaced on Sunday showing armed fighters from the Islamist militia group Libyan Dawn partying inside a captured U.S. diplomatic compound in Tripoli. The footage showed one fighter diving into a pool from a second-story balcony at the facility.

Tripoli airport and at least seven aircraft were reported damaged during fighting that began in July. Photos of the airport in the aftermath showed a number of damaged aircraft. The airport has been closed since mid-July.

The state-owned Libyan Airlines fleet until this summer included 14 passenger and cargo jetliners, including seven Airbus 320s, one Airbus 330, two French ATR-42 turboprop aircraft, and four Bombardier CJR-900s. Libyan state-owned Afriqiyah Airways fleet is made up of 13 aircraft, including three Airbus 319s, seven Airbus 320s, two Airbus 330s, and one Airbus 340.

The aircraft were reportedly taken in late August following the takeover of Tripoli International Airport, located about 20 miles south of the capital, by Libyan Dawn.

Al Jazeera television reported in late August that western intelligence reports had warned of terror threats to the region from 11 stolen commercial jets.

In response, Tunisia stopped flights from other Libyan airports at Tripoli, Sirte, and Misrata over concerns that jets from those airports could be on suicide missions.

Egypt’s government also halted flights to and from Libya.

Military forces in North Africa, including those from Morocco, Algeria, Tunisia, and Egypt have been placed on heightened alert as a result of intelligence warning of the stolen aircraft.

Egyptian military jets reportedly have conducted strikes inside Libya against Libyan Dawn positions recently, and U.S. officials said there are signs a larger Egyptian military incursion is being planned.

Egyptian President Abdel-Fatah al-Sisi was quoted as denying Egyptian air strikes into Libya have taken place but suggested that military action is being considered.

Secretary of State John Kerry last week told his Egyptian counterpart that the United States would speed up the delivery of Apache attack helicopters, although it is not clear the Apaches would be used in any Libyan operations.

Egypt’s military-backed government appears to be seeking a more significant role in regional security after the Obama administration helped engineer the ouster of Libyan strongman Moammar Qaddafi in 2011. Since then, the Obama administration, through its announced policy of "leading from behind," has stood by while Libya gradually has spiraled into chaos.

The Libyan government announced Sunday that it no longer controlled the capital of Tripoli.

"We announce that the majority of the ministries, institutions, and associations in the capital Tripoli are no longer under its control," a government statement said.

Libya’s parliament in August declared both Ansar al Sharia and Libyan Dawn as terrorist organizations working to overthrow the government.

Ansar al Sharia, which is based in Benghazi, recently publicized on social media that it has obtained large numbers of more sophisticated weapons, including SA-6 surface-to-air missiles, anti-aircraft guns, rocket-propelled grenades, shoulder-fired anti-aircraft missiles, assault rifles, and armored vehicles. The group is closely aligned with al Qaeda-linked rebels in Syria.

Abderrahmane Mekkaoui, a Moroccan military expert, told Al Jazeera television, which first reported the airline theft Aug. 21, the alert regarding the stolen jetliners was preventive and covers the region from Cairo to Lagos Nigeria.

Mekkaoui said the jets are being held by the Libyan group called Masked Men Brigade, which was designated as a foreign terrorist organization by the State Department in December.

The Masked Men Brigade is linked to al Qaeda and Ansar al Sharia—the group behind the Benghazi terrorist attacks of Sept. 11, 2012.

Until the Libya Dawn takeover of the airport, announced Aug. 24, two other militia groups, known as Al Qaqa and Al Sawa controlled the airport and all aircraft belonging to Libyan Airlines and Afriqiyah Airways.

Mekkaoui said "credible intelligence" reports given to states in the region indicated the Masked Men Brigade "is plotting to use the planes in attacks on a Maghreb state" on the 9/11 anniversary.

Counterterrorism expert Sebastian Gorka said that if the theft is confirmed, the stolen aircraft could be used in at least two ways.

"The first would be how commercial airliners were used on Sept. 11, 2001, literally turning an innocent mode of mass transit into a super-high precision guided missile of immense potency," said Gorka, who holds the Maj. Gen. Charles Horner chair at Marine Corps University in Quantico, Va.

"The second tactic could be to use the airframe with its civilian markings as a tool of deception to insert a full payload of armed terrorists into a locale that otherwise is always open to commercial carriers," he said.

Michael Rubin, a counterterrorism specialist with the American Enterprise Institute, said commercial jetliners in the hands of terrorists could be formidable weapons.

"Who needs ballistic missiles when you have passenger planes? Even empty, but loaded up with fuel they can be as devastating," Rubin said.

"Each plane could, if deployed by terrorists to maximum devastating effect, represent 1,000 civilian casualties."

Among the potential targets are urban areas and economic targets, like Saudi Arabia’s oil fields.

"Anyone who has ever flown over Saudi Arabia at night can see refineries like Yanbu lit up like Christmas trees against the blackness of the desert," Rubin said. "One Saudi security officer once told me that they would only have about 90 seconds to shoot down a hijacked plane from the time it left international airspace to impact in one of the region’s most important refineries."

Rubin said in 2003 a Boeing 727 went missing in Africa fueling concerns about a terror attack on the U.S. consulate in Karachi.

"What is striking is that more than a decade later, the United States hasn’t taken the need to safeguard what are effectively giant guided missiles seriously," he said.

A former Libyan general, Khalifa Haftar, has been leading anti-Islamist forces. His group has access to Libyan air force MiG jets that have conducted strikes on Libyan Dawn positions in recent days. Haftar also has conducted military raids in Benghazi.

The United Nations Security Council on Aug. 27 announced plans for new sanctions on Libyan militias and terrorists. In a resolution the U.N. warned of the "growing presence of al Qaeda-linked terrorist groups and individuals operating in Libya."