A militant group known as the Khorasan was close to carrying out an attack against the West before it was targeted by U.S. airstrikes in Syria Monday night, the Pentagon says

"We've been watching this group closely for some time," army Lt.-Gen. William Mayville, the Pentagon's operations chief, told reporters at a briefing Tuesday.

"We believe the Khorasan group was nearing the execution phase of an attack either in Europe or the homeland. We know that the Khorasan group has attempted to recruit Westerners to serve as operatives or to infiltrate back into their homelands."

Mayville said the group is not focused on Syrian President Bashar al-Assad's regime, or on the Syrian people: "They are establishing roots in Syria in order to advance attacks against the West and the homeland."

Eight strikes were conducted against the Khorasan group, targeting training camps, an explosives and munitions production facility, a communication building and a command and control facility, in the area west of Aleppo.

Plot was tracked for months

Mayville said the Pentagon is still assessing the effectiveness of the strikes.

"It would premature to comment on the effects, we need to do a little bit more study," he said.

The Pentagon and the White House haven't shared further details about the plot they say was in the final phase of planning, but did say it is something they have been tracking for "many months."

"So for some time now we've been tracking plots to conduct attacks in the United States or Europe. We believe that that attack plotting was imminent, in that they had plans to conduct attacks external to Syria. We also believe that the Syrian regime was not able to take action against that threat," said Ben Rhodes, deputy national security adviser at the White House, to reporters travelling with President Barack Obama to New York City.

"So in addition to the strikes against ISIL, we took action against the Khorasan group to disrupt that plotting against the United States and Western targets," he said, using another name for ISIS.

Khorasan members are believed to be former al-Qaeda fighters from Afghanistan, Yemen and Pakistan who have now formed a base of operations in Syria. Unlike ISIS, Khorasan is not trying to seize territory to create an Islamic state called a caliphate; it's primarily concerned with recruiting Westerners to carry out attacks in Western countries.