WASHINGTON — As the United States begins what could be a lengthy military campaign against the Islamic State, intelligence and law enforcement officials said another Syrian group, led by a shadowy figure who was once among Osama bin Laden’s inner circle, posed a more direct threat to America and Europe.

American officials said that the group called Khorasan had emerged in the past year as the cell in Syria that may be the most intent on hitting the United States or its installations overseas with a terror attack. The officials said that the group is led by Muhsin al-Fadhli, a senior Qaeda operative who, according to the State Department, was so close to Bin Laden that he was among a small group of people who knew about the Sept. 11, 2001, attacks before they were launched.

There is almost no public information about the Khorasan group, which was described by several intelligence, law enforcement and military officials as being made up of Qaeda operatives from across the Middle East, South Asia and North Africa. Members of the cell are said to be particularly interested in devising terror plots using concealed explosives. It is unclear who, besides Mr. Fadhli, is part of the Khorasan group.

The director of national intelligence, James R. Clapper Jr., said on Thursday that “in terms of threat to the homeland, Khorasan may pose as much of a danger as the Islamic State.”