Hard-line Islamist extremists aligned to al Qaida may be plotting to use Syria as a springboard to mount a new wave of international terror attacks, a senior British commander has warned.
Lieutenant General Mark Carleton-Smith, the director of operations at the Ministry of Defence, said Jabhat al Nusra was the strongest remaining al Qaida (AQ) franchise and could have the potential to threaten UK national security.
Giving evidence to the Commons Defence Committee, he said the group - identified by some analysts as a more serious long-term threat than Islamic State (IS) - may be associated with an element of British foreign fighters in Syria.
While it is not currently being targeted by RAF warplanes engaged in operations against IS in Syria and Iraq, Gen Carleton-Smith said they were ready to act if it emerged there was a "direct and imminent" threat to UK national security.
"There is a real danger that it will remain an abscess in the system," he told the committee.
"It is probably the strongest AQ franchise globally. It has its stronghold in Idlib province. It is certainly a spoiler in the political process in Syria and it might represent a Petri dish that becomes a threat to UK national security."
He said the group - which first emerged in 2011 - was a mix of fighters concentrating on the civil war in Syria and those with a wider, international agenda.
"A significant proportion are Syrian-focused that provide a wider wrapping to those much more specifically AQ-aligned elements that might harbour the ambition to use Syria as a springboard for an international terrorist attack plan," he said.
"There is potentially an element of British foreign fighters associated with it," he said.
"We are not specifically targeting the al Nusra front at the moment although if we were to determine that if there was a very specific direct and imminent threat to UK national security we would be able to do so."
Defence Secretary Michael Fallon, announcing the deployment of another RAF Airseeker surveillance aircraft to the region, said there was now "real momentum" behind the Iraqi offensive against IS - also referred to as Daesh - backed by the US-led coalition.
"Clearly progress has been made. There is a real sense of momentum now of the Iraqi and Kurdish forces advancing. That needs to be sustained," he said.
"The United States has made very clear to the coalition that this is actually the moment to step up and they have asked all members of the coalition to look and see what more they can do."
He acknowledged that IS - which has already established a foothold in Libya - could seek to expand its operations internationally as it came under pressure in its Syrian and Iraqi heartlands.
"That is certainly possible that we see attacks increase elsewhere in proportion to the way that they are being diminished in their caliphate. We will have to continue to be extremely vigilant about that," he said.