US Averts 'Active Plotting Against Homeland' By Hitting Al Qaeda Cell Khorasan in Syria

WASHINGTON— -- [This report has been updated, 9/24.]

American airstrikes in Syria have taken out members of a shadowy al Qaeda unit known as the Khorasan Group who were planning "imminent" attacks against targets including the U.S., the Pentagon said today.

Pentagon spokesperson Rear Admiral John Kirby declined to go into specifics, but told ABC News' George Stephanopolous, "We had very good indications that this group, which is a very dangerous group, was plotting and planning imminent attacks against Western targets to include the U.S. homeland and it was on that basis that we struck targets, Khorasan targets inside Syria."

"We believe that the individuals that were plotting and planning it have been eliminated and we’re going to continue... to assess the effectiveness of our strikes going through today," Kirby said.

The Khorasan Group -- consisting of about 50 or so hardened fighters of mixed past and current jihadi affiliations -- has been holed up in Aleppo, Syria under the protection of al Qaeda's official wing in the country, Jabhat al-Nusra, developing cutting edge weapons of terror with the help of al Qaeda's Yemen affiliate to strike Western civilian aviation targets, according to a half-dozen officials with knowledge of the group who spoke to ABC News.

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The U.S. -- acting alone rather than with Arab coalition partners such as in the ISIS strikes -- undertook at least eight strikes on the Khorasan Group's hideouts Monday night in the Aleppo area west of the ISIS strongholds in Raqqa, which were hammered in the sudden air offensive, U.S. Central Command, which oversees operations in the Middle East, said in a statement.

The strikes on the Khorasan Group were made "to disrupt the imminent attack plotting against the United States and Western interests conducted by a network of seasoned al Qaeda veterans... who have established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations," Centcom said.

The relatively tiny group's potential threat to the U.S. homeland stemming from experiments with next-generation undetectable bombs -- consisting of non-metallic components -- made the massive airstrikes critically urgent to thwart a clear and present danger, several officials told ABC News.

"They are taking the knowledge of [Al Qaeda in the Arabian Peninsula] AQAP's master bombmaker and experimenting with their own designs for undetectable IEDs [Improvised Explosive Devices]," one senior counterterrorism official recently told ABC News regarding the Khorasan Group's threat.

Over the past year, the U.S. and European allies have tightened up airline security measures because of intelligence that terrorists were making new, nearly-undetectable improvised explosive devices within toothpaste tubes or with clothing dipped in liquid explosives, as ABC News has reported since mid-2013.