By Phil Stewart and Mark Hosenball

WASHINGTON, Sept 24 (Reuters) - The United States is still assessing whether or not Mohsin al-Fadhli, a senior figure of the al Qaeda-linked Khorasan group, was killed in a U.S. air strike in Syria, government officials said on Wednesday.

One U.S. official, speaking on condition of anonymity, said earlier on Wednesday: "We believe he is dead." But other officials said the extent of the damage caused to the Khorasan group by Tuesday's airstrike was uncertain and they could not independently confirm the death of Fadhli or others.

The Pentagon cautioned that any confirmation could take time. Spokesman Colonel Steve Warren said: "We don't have personnel on the ground to verify, so we're continuing to assess."

White House spokesman Josh Earnest said he could not confirm any reports of Fadhli's death. "The early indications are that the strikes were effective," he told MSNBC.

U.S. accounts of the Khorasan group, the attack on its base and on Fadhli himself have left open questions.

One U.S. source said aircraft had dropped "tons" of bombs on what the U.S. believed was the group's main base. Some officials said they feared that after U.S. media reports about the group it may have moved people and equipment from the area.

The United States said on Tuesday it had been watching the group for some time. "We believe the Khorasan group ... was nearing the execution phase of an attack either in Europe or the homeland," said Lieutenant General William Mayville, director of operations at the Joint Chiefs of Staff.

Other officials on Wednesday gave a different analysis, saying U.S. agencies saw the group putting together the kind of manpower and structure which could carry out a plot and that unless quick action were taken, it might be too late to stop it.

Paul Pillar, a former senior expert on the region for the CIA who examined the official statements said: "Evidently what was imminent was not the execution of some terrorist plot in the West but instead the possible beginning of planning for such a plot."

A 2012 State Department notice offering a $7 million reward for information on Fadhli's whereabouts said he was an al Qaeda financier close to al Qaeda founder Osama bin Laden and among the few who knew in advance about the Sept. 11, 2001, attacks.

National security adviser Susan Rice, in an interview with NBC on Wednesday, called Fadhli "a dangerous operative," but added that he "has nothing like the cachet of bin Laden."

Islamist militants on social media have said that 33-year-old Fadhli had been killed in the air raids and said they were mourning him. But U.S. officials said that militant groups have previously been known to issue inaccurate claims about the deaths of specific militants.

The air strikes against Khorasan followed lengthy surveillance of the group, U.S. officials said. They described it as a "network" of seasoned al Qaeda fighters with battlefield experience mostly in Pakistan and Afghanistan, now working in league with al Qaeda's Syrian affiliate, the Nusra Front.

They have used the chaos of Syria's civil war as cover to try to devise new hard-to-detect bombs and recruit foreign militants holding Western passports to return home and eventually carry out attacks, U.S. officials said.

Security agencies do not believe that Khorasan has yet managed to recruit fighters with western passports, they said. (Additional reporting by Susan Heavey; Editing by Bill Trott and Grant McCool)