As the United States has shifted its main counterterrorism focus to the Islamic State from Al Qaeda, intelligence officials say the Khorasan Group has emerged as the cell in Syria that may be the most intent on, and capable of, striking the United States or its Western allies with an organized terrorist attack.

Matthew G. Olsen, the former director of the National Counterterrorism Center, said the succession of strikes against Khorasan had significantly hurt its ability to plot attacks.

“It seem plausible that the deaths of so many of these guys in Syria over the past year has really degraded the Khorasan Group as a whole,” Mr. Olsen said. The American effort has also helped prevent Al Qaeda from taking advantage of the chaos in Syria to find space in which to operate, he said.

There is little public information about the Khorasan Group, which American officials say is made up of about two dozen seasoned Qaeda operatives from the Middle East, South Asia and North Africa who were sent to Syria by Ayman al-Zawahri, Al Qaeda’s leader who is believed to be in Pakistan. Embedded within the Nusra Front, the Khorasan operatives were to recruit Europeans and Americans whose passports allow them to travel on American-bound jetliners with less scrutiny from security officials.

Micah Zenko, a scholar at the Council on Foreign Relations, said experts inside and outside the government were divided over the long-term efficacy of a so-called decapitation strategy, in which a terrorist group’s leaders are killed.

“There’s a pretty vigorous debate within the security community about whether this works,” Mr. Zenko said. Some officials believe that the fear of strikes makes it harder for extremists to meet and communicate and that killing leaders means advancing less competent operatives.

But Mr. Zenko said he was among the skeptics. The decapitation approach, he said, really becomes a “recapitation” strategy because the targeted groups learn to anticipate the deaths of leaders and prepare deputies to succeed them. Any disruption tends to be temporary, he said.