DOWNING Street has backed the m minister who said the only way of dealing with British Islamic State fighters in Syria was to kill them in almost every case.

Rory Stewart, the International Development minister, said converts to the terror group believed in an "extremely hateful doctrine", saying fighters can expect to be killed given the threat they pose to British security.

Hundreds of British citizens are known to have travelled to Syria to fight with Islamist groups during the course of the six-year conflict.

Brett McGurk, a top US envoy for the coalition fighting Islamic State, has said his mission is to ensure every foreign IS fighter in Syria dies in Syria.

Mr Stewart was asked about the comments on BBC Radio 5 Live's Pienaar's Politics when he said they were "very difficult moral issues", adding: "These are people who have essentially moved away from any kind of allegiance towards the British Government.

"They are absolutely dedicated, as members of the Islamic State, towards the creation of a caliphate, they believe in an extremely hateful doctrine, which involves killing themselves, killing others and trying to use violence and brutality to create an eighth century or seventh century state.

"So I'm afraid we have to be serious about the fact these people are a serious danger to us and, unfortunately, the only way of dealing with them will be, in almost every case, to kill them."

Mr Stewart, a former diplomat and Black Watch officer, went on to say: "These are people who are executing people in the back of their heads, who have held women and children hostage, who are torturing and murdering, trying by violence to impose their will.

"Our response has to be, when somebody does that, I'm afraid, to deal with that."

No 10 said Mr Stewart's comments were "consistent" with the Government's "determination to fight and defeat Daesh terrorists regardless of their nationality".

Theresa May’s spokesman said: "In relation to people who have taken the decision to go and fight for Daesh in Syria or Iraq, as the Defence Secretary said, you have made yourself a legitimate target."

For Britons who returned to the UK from the war zone, he added: “Everybody can expect to be investigated and where there is evidence to prosecute that prosecution by the CPS should take place."

In October, Sir Michael Fallon, speaking about British IS fighters in Syria and Iraq, said: "You have made yourself a legitimate target and run the risk, every hour of every day, of being on the wrong end of an RAF or USAF missile."

A Ministry of Defence spokesman explained: "Nobody should be in any doubt about our determination to fight and defeat Daesh terrorists, regardless of nationality.

"Anyone who does travel to Syria, for whatever reason, is putting themselves in considerable danger - particularly if they are fighting for our enemies."

Mr Stewart, a former diplomat, also said British authorities had made it "very clear" that people should not be volunteering with militia groups to fight IS.

"If you wish to serve your country and you wish to fight terrorism, then please apply to join the military or join the police or join our intelligence services, we'll train you, we'll work with you to do it in a legal and controlled fashion," he said.

The minister, who was raised in Scotland and whose family seat is Broich House near Crieff in Perthshire, also confirmed it remained British policy to remove Syrian president Bashar Assad.

"I don't think anyone should be in any doubt that the control that he has is brutal and ultimately fragile.

"The policy of the British Government is that Bashar al-Assad needs to step aside and we need transition to a new government, because so long as that man is in power, it is going to be impossible to have a long-term, stable, sustainable future for Syria," added Mr Stewart.