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(CNN) – U.S. strikes across Syria early Tuesday targeted ISIS, the terrorist group responsible for beheading two American journalists, and waging a terror campaign across vast swaths of Iraq and Syria.
But another target was the relatively unknown Khorasan Group - a collection of senior al Qaeda members who have moved into Syria.
"This is a group of very knowledgeable, experienced al Qaeda fighters," Sen. Marco Rubio, R-Florida, said in an interview with CNN's "The Lead with Jake Tapper."
Khorasan is in Syria not because it wants to overthrow Syrian President Bashar al-Assad. Rather, it is using the country as its operational space, taking advantage of the fact that the Syrian government is not trying to push it out, said Rubio.
"Anytime there's a vacuum created anywhere in the Middle East, it becomes a magnet for these sorts of terrorist groups to come in and operate from. They're a very serious threat," Rubio, a member of the Senate intelligence committee, said.
The group was actively plotting against a U.S. homeland target and Western targets, a senior U.S. official told CNN on Tuesday. The United States hoped to surprise the group by mixing strikes against it with strikes against ISIS targets.
The official said the group posed an "imminent" threat. Another U.S. official later said the threat was not imminent in the sense that there were no known targets or attacks expected in the next few weeks.
A few months ago, most Americans had never heard of ISIS, or Khorasan, sparking concerns of just how many more splinter al Qaeda groups are out there.
"It's a real risk," said Rubio, noting that Libya, a country that has increasingly become an ungoverned space, will likely see emerging terrorist groups, and that al Qaeda recently announced a new affiliate group in the Indian peninsula.
"This is going to be an ongoing issue," said Rubio. "These groups have splintered, but in many instances they coordinate with one another."
The Florida senator said he is hopeful that local ground troops in Iraq and Syria will be able to defeat ISIS and Khorasan. But echoing several top military leaders, Rubio said U.S. ground troops should not be taken off the table.
"The chances of local forces alone being able to defeat ISIL, or any group for that matter on the ground is dubious at best," he said.
"It's important for the President to be honest with the American people that at some point in the future, this might require some element of U.S. ground power in order to finish the job," said Rubio.