A little-known Al-Qaeda offshoot, the Khorasan group, found itself in the crosshairs of the United States on Tuesday as they prepared attacks on western targets from bases inside war-torn Syria.

With Islamic State jihadists terrorizing the region and seizing large chunks of Iraq and Syria, it came as a surprise to many that the Pentagon aimed its Tomahawk cruise missiles and other firepower not just at IS positions, but at a far smaller band of former Al-Qaeda operatives in northwestern Syria who had largely operated in secret.

The group's under-the-radar status was obliterated overnight. The Pentagon accused Khorasan of planning "major attacks" against the West, and said it eliminated the group's militants who were in the "final stages" of their plots to wreak havoc against Europe or the United States.

The group -- led by a former Osama bin Laden cohort -- is not new, experts and President Barack Obama's administration said.

Deputy National Security Advisor Ben Rhodes described them as including "core Al-Qaeda operatives from Afghanistan and Pakistan who made their way to Syria," and that the Pentagon acted because they believed attacks against western targets were "imminent."

The Pentagon said in the wake of US-led air strikes that it had been closely monitoring Khorasan "for several months."

And it stressed that a majority of the 40-plus Tomahawk cruise missiles launched into Syria were aimed at the Khorasan lair and training camps near Aleppo.

Action against Khorasan had long been considered and was "separate and apart from the growing threat from ISIL," a senior US official said, using one of several names for IS.

Matthew Henman, who heads the IHS Jane's Terrorism and Insurgency Center, said Khorasan's warriors slipped into Syria to link up with the Al-Nusra Front, which is Al-Qaeda's affiliate in the country.

Its funding and weaponry are limited, but Khorasan came "to exploit the vacuum and the very chaotic situation in Syria," Northeastern University professor Max Abrahms, who is also a terrorism analyst at the Council on Foreign Relations, told AFP.

- 'Like a hyena' -

By preying on the chaos and destruction in Syria, and less interested than IS jihadists in seizing territory, "Khorasan is like a hyena which basically picks up pieces of carcasses" in a brutalized landscape, Abrahms said.

Its maximum 1,000 members pale in comparison with the estimated 31,000 Islamic State fighters.

Khorasan leader Muhsin al-Fadhli is believed to have had close ties to Al-Qaeda founder bin Laden, and the group should be considered an Al-Qaeda affiliate "in more international-minded terms" than IS, according to Abrahms.

"The fact that the US targeted the Khorasan group in the very first wave of attacks really underscores how seriously the US takes this group," he said.

In brief remarks Tuesday Obama bluntly justified attacking Khorasan bases: "We will not tolerate safe havens for terrorists who threaten our people."

Lieutenant General William Mayville, director of operations for the US Joint Chiefs of Staff, said the group "is clearly not focused on the Syrian people" or the regime of strongman Bashar al-Assad.

"They are establishing roots in Syria in order to advance attacks against the West," he said, warning that Khorasan was recruiting foreign fighters who could use their western passports to return to their countries and carry out attacks.

"It's nothing new that Al-Qaeda wants to attack the US," said Aron Lund, editor of the Syria in Crisis website run by the Carnegie Endowment for International Peace.

"The fact that news about this Al-Qaeda-run anti-western cell linked to Al-Nusra Front emerged just over a week ago, through US intelligence leaks -- it's certainly an interesting coincidence," he told AFP.

Britain-based Syrian Observatory for Human Rights said some 50 Al-Qaeda militants -- presumably linked to Khorasan -- were killed in the air strikes, in addition to 70 IS jihadists.