In recent months Isis – now rebranded as Islamic State – has appalled the world with its brutal exploits, which are broadcast on social media. But according to US defense officials there is another group, about which much less is known, that poses a more direct threat to the US.
Washington DC officials told the New York Times that terror group 'Khorasan' had the US in its targets.
Led by Muhsin al-Fadhli, a senior al-Qaida operative who was close to Osama bin Laden, the Syria-based group is believed to be made up of battle-hardened al-Qaida members from the Middle East, South Asia and North Africa.
AP reports that the group is working with al-Qaida affiliates in Yemen to plot bomb attacks on US aviation. The reason the group set up in Syria was not to become involved in the country's civil war, but to recruit Americans and Europeans whose passports would allow them to board airliners easily.
The US director of national intelligence, James R Clapper, told the newspaper that "in terms of threat to the homeland, Khorasan may pose as much of a danger as the Islamic State".
It is believed that the recent ban on cell phones and laptops on flights to the US from Europe and the Middle East was imposed following intelligence on Khorasan's activities.
$7 million bounty
A Kuwaiti national, al-Fadhli is believed to be one of the few people who was trusted enough to know about the 9/11 attacks before they were carried out. He is believed to have moved to Iran after the US invasion of Afghanistan, where in 2012 the US State Department described him as the group's main operative.
He has a $7m (£4.3m) bounty on his head, and is believed to have links to wealthy Gulf State backers.
Some experts believe that Islamic State is more focussed on consolidating its territorial gains in Iraq and Syria that planning strikes against the West.
The chaos in Syria has led to it becoming a haven for Islamic extremist and radicals, where new groups are using it as a base to plan attacks.
"What you have is a growing body of extremists from around the world who are coming in and taking advantage of the ungoverned areas and creating informal ad hoc groups that are not directly aligned with Isis or Nusra," a former senior law enforcement official said.
Khorasan is believed to be a splinter group of Jabhat al Nusra, a Syrian jihadist group in Syria which is backed by al-Qaida.
Khorasan's group's name refers to a part of the old Islamic caliphate that included Afghanistan.