An al-Qaeda linked terrorist group targeted by the US in a second round of air strikes had been working with bomb makers from Yemen to produce explosives that could pass airport security undetected, including toothpaste tube bombs, an intelligence official has claimed.

The Pentagon said the Khorasan Group were plotting an “imminent attack against America and Western interests” when training camps, an explosives and munitions factory, a communication building and command and control facilities associated with the cell were attacked in eight strikes in the west of Aleppo.

The group is led by Muhsin al-Fadhli who has experienced a meteoric rise through al-Qaeda ranks. The US State Department put out a $7 million reward for information about his location in 2012.

In a briefing after the strikes, Lt. Gen. William Mayville, who directs operations for the Joint Chiefs of Staff, said the Khorasan Group was nearing "the execution phase of an attack either in Europe or the (US) homeland”.

US intelligence officials quoted by the Associated Press say the group has been working with bomb makers from al-Qaeda's Yemen affiliate to perfect explosives that can fool Western airport security measures, including a bomb in a toothpaste tube.

Meanwhile, another intelligence source told CNN plots also potentially involved bombs made of a non-metallic device or clothes dipped in explosive material. A plot involving concealed bombs on airplanes "was just one option they were looking at", the source told the network.

AP first reported on 13 September that US intelligence officials had identified the Khorasan Group as a serious threat, in part because the group had been working with bomb makers to test new explosive devices that could go undetected by airport security.

However, two US other officials said there was no particular location or target that had come to the attention of American intelligence agencies.

In a statement given after the US launched its first air strikes in Syria on Tuesday, Mr Obama described the group as "seasoned al-Qaeda operatives” whose members have come from Pakistan and Afghanistan and established a safe haven in Syria.

Mr Obama told a press conference: "Once again, it must be clear to anyone who would plot against America and try to do Americans harm that we will not tolerate safe havens for terrorists who threaten our people.”