WASHINGTON (AP) — Senior U.S. officials offered a more nuanced picture Thursday of the threat they believe is posed by an al-Qaida cell in Syria targeted in military strikes this week, even as they defended the decision to attack the militants.

James Comey, the FBI director, and Rear Adm. John Kirby, the Pentagon spokesman, each acknowledged that the U.S. did not have precise intelligence about where or when the cell, known as the Khorasan Group, would attempt to strike a Western target.

But both officials said that did not matter, because the U.S. became convinced that the militants had developed the plans, intentions and capability to kill Westerners. U.S. officials say the most likely scenario was an attempt to blow up a U.S. or European airliner in flight.

Syria is a place where "we don't have complete visibility," Comey told reporters at FBI headquarters, but "what I could see concerned me very much that they were working toward an attack."

Comey added, "It's hard to say whether that's tomorrow, three weeks from now or three months from now. But it's the kind of threat you have to operate under the assumption that it is tomorrow."

The U.S. has no open embassy or troops on the ground in Syria because of its brutal, three-year civil war, and the CIA's ability to operate there is limited.

Kirby, briefing reporters at the Pentagon, said, "I don't know that we can pin that down to a day or month or week or six months. It doesn't matter. Far better to be 'left of a boom' than to the right of it."

Kirby was using a military expression referring to a left-to-right timeline associated with efforts to stop roadside bombs before they explode.

"We can have this debate about whether it was valid to hit them or not, or whether it was too soon or too late," Kirby added. "We hit them. And I don't think we need to throw up a dossier here to prove that these are bad dudes."

The Khorasan Group, first revealed as a top U.S. terror concern in a Sept. 13 Associated Press story, comprises a small group of al-Qaida veterans from Afghanistan and Pakistan, along with "some newcomers" from other parts of the world, Comey said.

"That group was at the top of my list of things that I worry about," he said, and it remains so.

While Kirby said the strikes on eight Khorasan Group targets near Aleppo, Syria, were effective, he was not able to identify who was killed. He and Comey said they could not be sure the group's plots had been disrupted.

Kirby and Comey each were responding to questions about the use by senior American officials earlier this week of the term "imminent," about the threat posed by the Khorasan Group. Several U.S. officials told reporters this week that the group was in the final stages of planning an attack on the West, leaving the impression that such an attack was about to happen.

"I don't know exactly what that word means," the FBI director, a lawyer, said of "imminent." ''In this business, given the nature of the people involved, and what we could see, we assumed and acted as if (the attack would come) tomorrow."

U.S. intelligence officials have said the group was testing non-metallic explosives at its Syrian base that were designed to be slipped past airport security. One design included a bomb in a toothpaste tube. The threats led the Transportation Security Administration to impose new security rules, including a ban on toothpaste tubes, in February on flights to Russia, and a ban on uncharged laptops and cellphones in July on flights originating in Africa, Europe and the Middle East.

Despite the worries about Khorasan militants who have been key al-Qaida figures for years, Comey said he continues to believe that Americans are safer because what some analysts call "core" al-Qaida has been significantly degraded in Afghanistan and Pakistan.

A U.S. intelligence assessment that al-Qaida is no longer capable of pulling off a complex, 9/11-style attack remains true, he said. But the proliferation of al-Qaida affiliates and the use of the Internet for radicalization has spread the danger farther afield.

"We're safer, but that doesn't mean we're safe," he said. "The metastasis of (al-Qaida) has made it harder to track."

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Associated Press writer Lolita C. Baldor contributed to this report.

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Follow Ken Dilanian on Twitter at https://twitter.com/KenDilanianAP