US officials are "still assessing" the impact of strikes by US aircraft that entered Syrian air space on Tuesday, but giving so-called Islamic State targets a wide berth, they dropped their loads on the little known Khorasan group - and while they refuse to be categoric, US officials are surmising that group leader Muhsin al-Fadhli was among the dead.

"We believe he's dead," an unnamed administration official was quoted. "[But] we just don't have a confirmation to make at this point," added Pentagon spokesman Colonel Steve Warren.

Intriguingly, the US mirrored Khorasan tactics in mounting the surprise attack – just as the group is in Syria to take advantage of the civil war chaos and to recruit fighters who carry Western passports, American counter-terrorism forces used the opening assault on IS in Syria as cover to go after Khorasan.

In backgrounding reporters, US officials have sketched only the vague outlines of a group led by one of Osama bin Laden's most trusted comrades – at age 19, Fadhli reportedly was privy to plotting for the 9/11 attacks on the US and 13 years on had been dispatched to Syria by bin Laden's successor, Ayman al-Zawahiri, to recruit foreign fighters whose travel documents would let them travel easily in Europe and the US.

In what is thought to be the first public mention of the group by a US official, just last week, Director of National Intelligence James Clapper, said that "in terms of threat to the [US] homeland, Khorasan may pose as much of a danger " as IS.

The threat was that Khorasan was in Syria, but looking beyond – scouting for jihadis with the right travel papers and at the same time, working with al-Qaeda's bomb-making specialists in Yemen.

The Kuwaiti-born Fadhli won a mention in a George W. Bush speech in 2005; and in 2012, the State Department described him as a "senior facilitator and financier" as it put a US$7 million price on his head.

On Tuesday, US President Barack Obama made an oblique reference to "seasoned al-Qaeda operatives" in Syria – but in his September 10 "degrade and destroy" speech,  the president made no mention of this other threat that he planned to eliminate.

And just as there was no clarity on the death of Fadhli, which was being mentioned in jihadi social media posts; neither was there any news on the extent to which the attacks might have degraded or destroyed Fadhli's entire operation.

Reports by the United Nations, among others, say Fadhli fought in Afghanistan and Chechnya and for a time had operated from Iran. In Syria, he presided over a group of about 50 fighters, most of who had come from Afghanistan and Pakistan.

Pentagon spokesman Rear Admiral John Kirby described Tuesday's strikes as "very successful", as he ticked off the Khorasan targets near the northern city of Aleppo – "training camps, an explosives and munitions production facility, a communications building and control facilities".

Concern about the group's intentions was the reason for a tightening of security around flights to the US a few months earlier but, in describing why the group had to be attacked, accounts  by various officials don't quite gel.

Deputy national security adviser Ben Rhodes on Tuesday said Khorasan was engaged in actual plotting that was going on from Syria, but it
was unclear whether it was the planning or the attack that was "imminent".

A day later, another official briefing told reporters said that the group was "nearing the execution phase" for an attack "in Europe or in the [US] homeland". But as the Guardian pointed out, hours before the air strikes, another senior US official had said there was no indication of an imminent domestic threat from Khorasan.

Come Thursday, the Pentagon's chief spokesman insisted the group was "nearing the end stage of planning an attack on a western target".

And again, other unnamed officials quoted by The New York Times and The Washington Post were more cautious – it was unclear if Khorasan had actually selected targets, dispatched operatives or had even given the "go" command for a plot.

"Aspirational" was how one of the officials described Khorasan's planning.

The Pentagon's Rear Admiral Kirby was quoted: "I can't say with extreme confidence the we know we have in fact disrupted a specific attack, but we definitely know we have hit targets  that belong to them, were of use to them."