By Mark Hosenball

WASHINGTON, Oct 3 (Reuters) - U.S. airstrikes in Syria in September that were aimed at a faction of al Qaeda militants said to be plotting attacks against the West failed to deliver a decisive blow against them, U.S. officials familiar with the operation said late this week.

While U.S. intelligence agencies are still assessing the results of the Tomahawk cruise missile strikes, three U.S. officials said indications are that many suspected leaders and members of the Khorasan Group escaped, along with high-tech explosive devices they were said to be preparing to attack civil aviation or similar targets.

"They thought people were there but they were not there," said one U.S. official familiar with the Obama administration's plan.

This official and others spoke on condition of anonymity to discuss the Sept. 22 airstrikes, many details of which are classified.

The targets of the strikes were fighters from the Khorasan Group, which is how the U.S. government refers to a cell of al Qaeda veterans who had relocated to Syria from the Afghanistan-Pakistan border region.

At the time of the strikes, Pentagon spokesman Rear Adm. John Kirby said they were conducted to "disrupt imminent attack plotting against the United States and Western targets."

Federal Bureau of Investigations director James Comey told reporters on Sept. 25 that he was "not confident" that the plots against the United States had been disrupted.

A White House spokesman declined to comment on the strikes' effectiveness.

In the aftermath of the strikes, U.S. officials have dialed back their warnings, saying that any planned attacks by Khorasan may not have been imminent.

The U.S. government still maintains that the group is sufficiently skilled and well armed to launch a surprise attack against the West.

The sources who spoke about the strikes said that since the raids apparently missed their main targets, Khorasan members are likely still actively planning attacks.

The U.S. attack on the Khorasan Group's base was part of the first night of air strikes that were launched in Syria last month by the United States and allies. Subsequent strikes have primarily targeted militants from Islamic State, which has seized territory in Syria and Iraq.

The Khorasan base targeted in the strikes was part of a larger encampment operated by the al Nusrah Front, al Qaeda's designated affiliate in the region, near the northern Syrian city of Aleppo, U.S. officials have said.

Some of the officials who disclosed the airstrikes' limited impact said they blamed news leaks for helping tip off Khorasan's leadership that it was in Washington's crosshairs.

U.S. media reports published in the two weeks before the strikes disclosed the American government's concern about Khorasan and Muhsin al Fadhli, one of the group's alleged leaders. Previously, their names had not been widely used in public.

"Discussion and speculation in the media about this group, Khorasan, tempered expectations for what could be accomplished in the strikes," said one of the sources, a U.S. intelligence official.

Since the U.S. attacks, conflicting reports have circulated on social media and elsewhere about whether al Fadhli was killed in the air strikes. U.S. officials said on Friday they were not sure if Fadhli was dead or if he had himself declared dead and was still operating in the shadows.

The U.S. officials described Khorasan as a group of a few dozen veteran militants who had fought with core al Qaeda forces in such places as Afghanistan and Pakistan.

The group recently relocated to al Nusrah-controlled territory in Syria because they regarded it as a safe haven where they would have time and space to hatch plots. Khorasan is playing no direct role in Syria's civil war, U.S. officials have said.

To carry out attacks outside Syria, Khorasan's members built or acquired explosives designed to evade Western security measures, including airport checkpoints, U.S. security and intelligence officials said.

In response, officials have said, U.S. authorities this summer publicly ordered stepped-up security checks on devices such as mobile phones and computers carried by passengers boarding international flights to the United States.

Where Khorasan members acquired sophisticated bomb-making skills is the subject of debate.

Some of the officials said the know-how came from al Qaeda's Yemen-based affiliate, which has produced innovative devices in the past, including bombs hidden in underwear and printer cartridges.

But other officials said that the Yemen affiliate, al Qaeda in the Arabian Peninsula, would not want to supply other groups with its most advanced technology because it wanted sole credit for designing and using it. (Reporting by Mark Hosenball; Editing by Warren Strobel, Andrew Hay, Toni Reinhold)