Media playback is unsupported on your device Media caption General Lord Richards: ''We need an army that can sustain a demanding operation''

The former head of the armed forces has called for the British army to increase in size, in order to tackle Islamic State (IS) and other Muslim extremists.

General Lord Richards told the BBC "thousands" more soldiers were needed.

Deputy Prime Minister Nick Clegg said while IS was a "serious threat to our way of life", the response was not as straightforward as "ramping up" troops.

A video showing the apparent beheading of aid volunteer Alan Henning, 47, was released by IS on Friday.

He was the second British hostage to be murdered, following the death of David Haines.

'Back to drawing board'

It isn't actually a counter-terrorist operation, this is a conventional enemy General Lord Richards

Lord Richards said the plan to cut the regular army and bolster numbers using reservists could be made to work - but added that we were "in a different era" to just two or three years ago.

"This needs to be put right quickly," he said. "My instinct is it can't be and we need to go back to the drawing board and revise the size of the regular Army back up again.

"If we are talking about a generational struggle, as I think the prime minister is and he's absolutely right to talk about, then we need an army that can sustain a demanding operation, not just in Syria and Iraq potentially, in a largely support role, but think of all the other places where Muslim extremism is causing havoc."

He told the Andrew Marr Show he wanted the UK to play a role in the aerial campaign over Syria, describing the move as a "bit of a no-brainer".

'Scale of response'

"I'm very clear that that is the view of the prime minister and most of his party," said Lord Richards. "His problem is delivering the politics that enable us to do that.

"But air power alone will not win a campaign like this.

"It isn't actually a counter-terrorist operation, this is a conventional enemy."

The UK will either have to put "boots on the ground at some point" or "very energetically and aggressively train up those who will do that with us and for us".

He added: "My worry at the moment is that the scale of the challenge isn't being met by the right scale of response.

"I am worried that without more intensive use of Western boots on the ground in a support role then we won't do it in the time we need to get on and do this in."

Media playback is unsupported on your device Media caption Nick Clegg MP: "Increasingly what you're having is states co-operating together... against stateless, mobile groups"

Mr Clegg said it was not a case of increasing forces as Lord Richards suggested, however.

He told the show: "I don't think it's a question of simply ramping up conventional armed forces again as if we were fighting state-to-state conflicts.

"Increasingly what you are having is states co-operating together in a jigsaw operation where different countries bring to the effort different capabilities against stateless mobile groups."

The Lib Dem leader said he would not advocate extending the current air campaign over Iraq into Syria, adding: "We need to persuade the British people and Parliament as well that every time we put servicemen and women in harm's way, we do so with a clear strategy in mind, and I think the combination of air strikes and ground forces needs to be properly thought through."

'Large sums of money'

Senior military figures have said the UK government must put pressure on Gulf states not to fund terrorist groups.

Former head of the British army Lord Dannatt told the Sunday Telegraph those regimes had to put a stop to fundraising terror networks before they could invest in the UK.

He said: "It is completely unacceptable that some individuals in Qatar, Saudi Arabia and elsewhere see advantage in channelling large sums of money to the so-called Islamic State."

Former defence secretary Liam Fox told the newspaper: "Isil is well funded. Money has been flowing from rich individuals in the Gulf states, if not their governments, to finance them and their Sunni allies in their battle against the Assad regime."

Lord Richards told the Marr Show this approach was "part of solution" but not a "total solution by any means".