Up until late last week, no US official had ever publicly mentioned the terrorist group known as the Khorasan. On Monday night, the US carried out unilateral airstrikes against the previously unknown group in northwest Syria.

And on Tuesday, US officials were describing the group as an imminent threat on par with or worse than the group calling itself the Islamic State (also known as ISIS or ISIL), which has been the focus of US airstrikes for more than six weeks.

The key difference between ISIS and Khorasan: US intelligence believes Khorasan poses a threat to the US and its homeland, while it believes ISIS does not currently have the capability to carry out a large-scale attack on the US homeland.

Khorasan was involved in "imminent attack plotting" against the US and its interests along with Europe, the Pentagon said Tuesday. The group has been portrayed as a collection of top Al Qaeda officials from Central Asia who have been taking advantage of the chaos in Syria to establish training camps. In a statement from the White House, US President Barack Obama called them "seasoned" Al Qaeda operatives.

"The intelligence reports indicated that the Khorasan Group was in the final stages of plans to execute major attacks against Western targets and potentially the U.S. homeland," Lt. Gen. William Mayville, the director of operations for the Joint Chiefs of Staff, told reporters at the Pentagon on Tuesday.

BuzzFeed's Rosie Gray reports that Khorasan appears to have been connected to Ibrahim al-Asiri, the master bomb maker in Al Qaeda in the Arabian Peninsula (AQAP). The group is believed to be led by Muhsin al-Fadhli, a senior Al Qaeda operative and a close confidant of Osama bin Laden.

Fadhli had previously headed Al Qaeda's operations in Iran. However, he is believed to have been relocated to Syria in mid-2013 when he joined up with Jabhat al-Nusra, the official Al Qaeda franchise in Syria.

resident Barack Obama delivers a statement on the recent airstrikes against ISIS on the South Lawn of the White House. Win McNamee/Getty Images

This conflation of Al Qaeda members and franchises in Syria has led to debate about whether Khorasan is an independent Al Qaeda franchise or a subset of Nusra. Aaron Zelin, a Richard Borow Fellow at the Washington Institute think tank and the founder of Jihadology, believes that the two groups are one in the same.

It’s cute Pentagon is literally making up new group called ‘Khurasan’ when it’s just AQ AfPak/Iran guys in JN. Don’t get my gov sometimes. — Aaron Y. Zelin (@azelin) September 23, 2014 I find it absurd that the DOD doesn’t think people can see through its Khurasan/JN IO. Just say what you’re doing. This is 2014. — Aaron Y. Zelin (@azelin) September 23, 2014

Thomas Joscelyn, an editor of The Long War Journal and scholar at the Foundation for Defense of Democracies, told Business Insider that Khorasan consists of members of "core Al Qaeda" who were dispatched to Syria by Ayman Al Zawahiri, Osama Bin Laden's successor as head of the international jihadist group.

"These are basically senior operational guys within Al Qaeda who are operating in Syria, and using the fertile recruiting ground of Syria to identify potential terrorist operations in the west," Joscelyn told Business Insider. The operatives are embedded within Nusra's leadership structure, and take their name from the Khorasan Shura, an advisory council within Al Qaeda central.

Senior Obama administration officials said the group's operatives have fought in Afghanistan and Pakistan. The group was portrayed as a known threat, one that a senior US official said had forced the US to contemplate taking action for quite some time.

The official said action against Khorasan was being contemplated separate from ISIS threat "for months" and likely would have been taken anyway.

"This is essentially the same cast of characters that we've had our eye on for years," the senior US official said. "These are known operatives to us. This is more like a group of people seeking to exploit the safe haven in Syria to plot against us. And that's why we took the action we did."

US Central Command said strikes against the group northwest of Aleppo in Syria targeted training camps, an explosives and munitions production facility, a communication building, and command and control facilities.

Armin Rosen contributed to this report.