What is the Khorasan group?

In addition to the airstrikes launched Tuesday against the Islamic State in Iraq and the Levant (ISIL) in Syria, the United States also targeted what it referred to as the Khorasan group, which the Pentagon said was plotting an "imminent attack" against the U.S. and Western interests. 

American warplanes launched eight strikes west of Aleppo aimed at the Khorasan fighters, whom U.S. Central Command described as a "network of seasoned Al-Qaeda veterans" from Afghanistan and Pakistan that have "established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices." 

Just last week Director of National Intelligence James Clapper told The New York Times that "in terms of threat to the homeland, Khorasan may pose as much of a danger as the Islamic State." 

But who or what is Khorasan? 

Citing "officials with knowledge of the group," ABC News reported Tuesday that it is believed to have about 50 members who have been in hiding in Aleppo — the largest city in Syria — under the protection of Al-Qaeda’s Syrian branch, a rebel group known as Jabhat al-Nusra.

But the Khorasan fighters did not go to Syria principally to join the fight the government of President Bashar al-Assad in the country’s long-running civil war, U.S. officials said. Instead, they said the Khorasan members were sent by Al-Qaeda leader Ayman al-Zawahiri to recruit Europeans and Americans whose passports allow them to board a U.S.-bound airliner with less scrutiny from security officials.

U.S. officials view the group as a threat because it has been working with Al-Qaeda in the Arabian Peninsula (AQAP), Al-Qaeda's Yemen affiliate, on testing explosive devices that cannot be detected by airport security in Western countries, The Associated Press reported earlier this month. A senior Obama administration official told the AP that the Transportation Security Administration's (TSA) decision in July to ban uncharged laptops and cellphones from certain flights, along with other enhanced security measures, were prompted by the Khorasan threat. The same official also confirmed to the AP that the group had been testing bombs at its camps in Syria. 

The group is led by Muhsin al-Fadhli, a Kuwaiti who spent time in Iran and has long been identified as a significant Al-Qaeda player, a U.S. official briefed on intelligence reports told the AP. 

Lt. Gen. William Mayville, director of operations for the Joint Chiefs of Staff, told reporters Tuesday morning that the U.S. believes the Khorasan group "was nearing the execution phase of an attack in Europe or on the homeland." He did not provide details or answer questions about whether officials believed the threat was ongoing. 

The U.S. Central Command said Tuesday that the strikes against Khorasan — which were undertaken unilaterally, unlike the coordination with Arab countries during the strikes on ISIL — targeted the group's training camps, an explosives and munitions production facility, a communication building, and command and control facilities.

But Aymenn Jawad Al-Tamimi, a fellow at the Middle East Forum, a conservative-leaning think tank based in Washinton, D.C, told Al Jazeera that Khorasan is "not actually a separate group," but rather a contingent of Al-Qaeda members within Jabhat al-Nusra that come from the Afghanistan-Pakistan border region. Khorasan is a reference to a province under an ancient Islamic caliphate that once included parts of Afghanistan.

"The U.S. is using this distinction as a means to strike Jabhat al-Nusra, which fits in with a wider policy in the region of targeting Al-Qaeda affiliates with airstrikes, while hoping to avoid alienating locals and other rebel groups who support or wish to work with Jabhat al-Nusra," Al-Tamimi said. 

The U.S. and its Arab allies launched more than 200 airstrikes against roughly a dozen targets in Syria during the assault that began early Tuesday, a U.S. official told the AP. A majority of the missiles fired from Navy ships targeted the Khorasan Group, said the official, who was not authorized to provide mission details so spoke on only condition of anonymity.

Al Jazeera and The Associated Press. Philip J. Victor contributed to this report.