WASHINGTON (CBSDC) – The terror attacks in Paris are just the latest in a series blamed on ISIS. The radical Islamic group has claimed responsibility for last week’s twin suicide bombs in Beirut that killed more than 40 people and the Russian jetliner crash over Egypt that killed 224 people.

Former CIA deputy director and CBS News senior security contributor Michael Morell spoke to “CBS This Morning” to discuss the U.S. strategy against the terror group.

“What we saw in Paris was the manifestation of a year-long effort by ISIS to build an attack capability in Western Europe,” he said.

While French police search for suspects, it’s crucial that security be increased, warned Morell, especially in American cities.

“They’ve also said they are going do that here. So I do think it’s a matter of time before they make that effort,” he said. “And unless we degrade them, unless we push back on them, they will be successful.”

French police have identified more members of a sleeper cell said to be behind the Paris attacks.

Morell said the refugee crisis has made it more difficult to prevent terrorists from infiltrating Western nations.

“How do we screen these migrants coming into Europe and coming into the United States?” he asked.

On Sunday, French warplanes responded by attacking ISIS targets in Syria. The attacks increase the pressure on President Barack Obama to rethink American strategy against the terror group.

“On the offensive side, we have to look at rules of engagement how much collateral [damage] are we going to accept in the fight against them,” explained Morell. “We have to look at how many troops are we willing to put on the ground, not to do the combat themselves, but to be on the front lines to advise and assist. To be on the front lines to call in airstrikes.”

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