U.S. officials say airstrikes in Syria on Monday night were aimed in part at disrupting an “imminent attack” by Al Qaeda-tied fighters on either the United States or Europe, in the strongest evidence yet of the growing threat posed by the group.

The strikes against the so-called Khorasan Group, conducted only by the U.S., were separate from the strikes against the Islamic State in which Arab allies participated.

U.S. Central Command said eight strikes were launched against the group’s training camps, command-and-control facilities and other sites in the area west of Aleppo. 

"We will not tolerate safe havens for terrorists who threaten our people," President Obama said Tuesday, in reference to the strikes on the Khorasan Group. 

Fox News is told that the targeting reflects intelligence regarding the threat of militants trying to recruit western passport-holders to act as mules and carry non-metallic explosives on jets bound for the Europe and the U.S.

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Pentagon spokesman Rear Adm. John Kirby confirmed Tuesday that the plotting was far along.   

“This is a very dangerous group,” Kirby told Fox News. “We had information, good information that they were very actively plotting and very close to the end of that plotting -- and planning an attack on targets either in Europe or the U.S. homeland.”

As for the result of the airstrikes, he said: “We think we hit what we were aiming at.”

While most of the public attention has been focused on the Islamic State threat, the campaign to disrupt the Khorasan Group is emerging as a critical side mission.

The network is made up of Al Qaeda fighters from Afghanistan and Pakistan – including a half-dozen operatives associated with Al Qaeda senior leadership, Fox News has learned -- who came to Syria to develop new international plots.

A former Pentagon official said that among them is a top strategist and finance chief who previously helped run Al Qaeda’s network in Iran.

Until recently, U.S. officials did not even speak openly about the group. But last week, Director of National Intelligence James Clapper confirmed publicly that the network was operating in the region and said they may pose as great a threat to the U.S. as the Islamic State.

“There is potentially yet another threat to the homeland, yes,” Clapper said at an intelligence conference in Washington last week.

U.S. officials say they want to link up with the Al Qaeda affiliate known as the al-Nusra Front. Their members are said to include operatives trained by notorious bombmaker Ibrahim al-Asiri, thought to be behind the 2009 attempted bombing of a Detroit-bound jet and another unsuccessful plot in 2010.

In July, information related to the Khorasan Group and fresh plotting led the TSA to change its security posture and call for increased scrutiny of cell phones and laptops on U.S.-bound flights. 

Lt. Gen. William Mayville Jr., director of operations for the Joint Chiefs of Staff, said at a briefing Tuesday that officials are "still assessing the effects" of the Syria strikes but said Khorasan was nearing the "execution phase" of an attack. 

He said the fighters are "clearly not focused on the Assad regime" or the Syrian people, but rather taking root in Syria to plot attacks against the West. 

U.S. Central Command, in a statement overnight, described it as a “network of seasoned Al Qaeda veterans.” Central Command said the group has “established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations.”

Fox News’ Catherine Herridge contributed to this report.