DAVID Cameron has hit back at a highly critical MPs' report over the UK's level of involvement in fighting Islamic State terrorists, insisting the Coalition Government was "doing everything we can to defeat the monstrosity of Isil".

The Prime Minister's response came as the United Nations warned it had received reports from Iraq of "several cases of mass executions of boys as well as reports of beheadings, crucifixions of children and burying children alive".

The UN agency urged that more should be done to counter the Isil threat, saying Iraq needed to "take all necessary actions to ensure the safety and protection of children and their families".

But military chiefs in Baghdad have pleaded for more weapons and training of troops as local forces struggle in their fight against the jihadists.

Earlier this week, the House of Commons Defence Committee condemned Britain's "tiny" contribution to the battle against the terrorists - also known as IS, Isis and Daesh - noting how it had undertaken just six per cent of airstrikes against them; the equivalent of one a day.

But Mr Cameron hit back, saying: "Look, we are taking action. We are the second nation in terms of bombing runs that have been carried out in Iraq. That is making a difference on the ground and we are seeing the growth of Iraqi security forces and Kurdish forces sweeping Isil out of parts of Iraq, and we need a long-term plan for both Iraq and Syria so we defeat Isil."

He stressed how fighting the terrorists was more than just military action, saying: "We have got to go after the hate preachers, we've got to go after the radicalisation. We have got to stop people travelling to Syria, we have got to prosecute those who have been and been radicalised and would do us harm."

The PM pointed out how there had been "big growth" in the number of people in Britain prosecuted and convicted for Islamist extremist violence. "We need to keep up our guard," he declared, adding: "The threat of Islamist extremism is a threat that we are going to face throughout this generation. It is going to be years rather than months."

Nick Clegg also rebutted some of the MPs' criticism, saying: "The suggestion...that somehow our commitment can be measured according to the number of boots on the ground in Baghdad is a misplaced measurement."

He said he did accept the importance of using British expertise to get Sunni tribes in Iraq to feel more comfortable with the administration in Baghdad, noting how at present they were disenchanted with their own government in Iraq. This, explained the Deputy Prime Minister, was "giving Isil the space to run riot and inflict their grotesque violence in large parts of Iraq right now".

In their report, MPs expressed shock that service chiefs did not provide a clear statement of the UK's objectives in Iraq when they gave evidence to them.

Rory Stewart, the committee's Conservative Chairman, said: "The nightmare of a jihadist state establishing across Syria and Iraq has finally been realised.

"Daesh controls territory equivalent to the size of the UK, has contributed to the displacement of millions, destabilising and threatening neighbouring states, and providing safe-haven to an estimated 20,000 foreign fighters, many dedicated to an international terrorist campaign. Yet, the role that the UK is playing in combating it, is strikingly modest."

The MPs discovered on a visit to Iraq in December that there were only three UK military personnel outside the Kurdish regions of the country compared with 400 Australians, 280 Italians and 300 Spanish.

The committee recommended Britain should invest heavily in staff to develop a better understanding of the situation on the ground and to help develop a realistic coalition plan for dealing with IS.