Who is ISIS-K and what terror threat do they pose to the United States?

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WASHINGTON, D.C. — The U.S. government is vowing revenge after one of the deadliest attacks on American troops in Afghanistan in years.

The terrorist group ISIS-K is claiming responsibility for two bombings in Kabul Thursday which killed 13 American servicemembers.

READ: Kabul airport attack: Evacuations resume as death toll tops 100 (live updates)

ISIS-K is a regional affiliate of the Islamic State.

National security expert Jamil N. Jaffer said they are considered a small to medium terrorist group with numbers in the thousands versus the tens of thousands.

He said the group has ties to Iraq and Syria and that ISIS-K has been fighting in various forms in Afghanistan for years but recently they’ve become more active.

“There is always a desire to attack the homeland. ISIS-K may not have the capability today to do that but they certainly have the desire and the intent,” said Jaffer. “So the question is when will they develop the capability? Can they work with their other franchises? Remember they’re part of this larger umbrella of the ISIS organization.”

READ: US presses on with evacuations despite fears of more attacks

Jaffer said there could be additional terror attacks from them as the evacuations continue in Kabul.

“They’re also seeing what they believe is an American retreat from the country, put aside the politics of it – that’s how they’re perceiving it as they see that it emboldens them to commit attacks in the U.S., in Europe and in Afghanistan against our allies that are there

The Pentagon says it is investigating what happened, so they know what to look for to prevent future attacks.

READ: ‘We can’t do enough’: Lawmakers flooded by evacuation pleas

Press Secretary John Kirby said they are preparing for future threats.

“I think the threat from ISIS is real and no one wants to see that threat grow,” said Kirby.

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