Experts were similarly dubious about the distinction.
"The name refers to al-Qaeda fighters previously based in Afghanistan, Pakistan, and Iran who have travelled to Syria to fight with ... Al-Nusra," said Matthew Henman, head of IHS Jane's Terrorism and Insurgency Centre. "They ... should not be considered a new or distinct group as such."
Aron Lund, editor of the Syria in Crisis website run by the Carnegie Endowment for International Peace, raised similar doubts.
"The fact that news about this al-Qaeda-run, anti-Western cell linked to Al-Nusra emerged just over a week ago, through US intelligence leaks - well, it's certainly an interesting coincidence," he told AFP.
"And it certainly helped make the case for attacking them, for why this mattered to US national security, and for why this was not about attacking a rebel group in Syria but about attacking a group hostile to the US."
Claims of a distinction are lost on many of Syria's rebels, who have also often rejected the world community's designation of Al-Nusra as a "terrorist" group.
When Washington added Al-Nusra to its list of "terrorist" organisations, even the internationally backed Syrian opposition National Coalition criticised the decision.
The Coalition's support for the group cooled after Al-Nusra officially pledged allegiance to al-Qaeda chief Ayman al-Zawahiri and was named the group's official Syrian branch.
But on the ground, almost all rebel groups have been willing to co-operate with Al-Nusra, seeing them as distinct from Isis, which espouses transnational goals and includes many non-Syrians among its ranks.
Al-Nusra, by comparison, has maintained a focus on overthrowing Syrian President Bashar al-Assad - the main goal of the rest of the Syrian opposition - and has mostly Syrian fighters.
The co-operation between moderate and Islamist rebels and Al-Nusra has even extended beyond the fight against Assad to the creation of a coalition that began fighting Isis jihadists in January.
Angered by its abuses against civilians and rivals, the loose coalition succeeded in pushing the group out of much of northern Syria, though it has since regained ground, bolstered by new recruits and weapons seized across the border in Iraq.
That history of co-operation has left some rebels and activists on the ground suspicious and even angry about the strikes on Al-Nusra.
Ibrahim al-Idlibi, an activist in Idlib province, said the opposition backed strikes against Isis, but not against Al-Nusra.
Al-Nusra "has stood with the rebels against both Daesh and the regime", he added, using the Arabic acronym for Isis. On social media questions have been raised about the mysterious Khorasan which the US says it is also targeting in Syria.
NBC News' chief foreign correspondent Richard Engel sent out a tweet saying: "Syrian activists telling us they've never heard of Khorasan or its leader." Al Jazeera English reporter Imran Khan wrote that the name was new to him and contacts across the Middle East and Asia.
"Khorasan is almost certainly a term that the US Government has coined," he wrote. "It's suitably exotic. Geographically, it's a historical region in the northeast of Iran and includes Afghanistan and what is now Pakistan. This tallies with what I've been told by my sources, and who the Americans claim, make up the group: a hardcore of former al-Qaeda fighters who come from Pakistan, Afghanistan and Iran."
He added: "My guess ... is that Khorasan is a term that may well have been coined by intelligence analysts that has been picked up by politicians and then an unquestioning US media that has turned it into a group that should be feared ... What it clearly isn't is a name that jihadists know or use. To that end, why would the US Government put the name out there? ... It pushes the idea that there are groups out there that operate in a shadowy manner."
- AFP, Herald staff
