BBC NewsIraq crisis: Cameron warns of possible IS threat to UKIslamic State militants could grow strong enough to target people on the streets of Britain unless action is taken, David Cameron has warned.The PM, writing in the Sunday telegraph, said a "humanitarian response" to IS was not enough and a "firm security response" was needed.
It comes as Church leaders expressed concern that the UK had no "coherent" approach to tackling Islamic extremism.
IS has seized large parts of northern Iraq and Syria over the summer.
The violence has so far driven an estimated 1.2 million Iraqis from their homes.
Kurdish forces, supported by US air strikes, are currently battling to retake Mosul dam from IS fighters in northern Iraq.
'Only grow stronger'Mr Cameron wrote: "True security will only be achieved if we use all our resources - aid, diplomacy, our military prowess - to help bring about a more stable world.
"If we do not act to stem the onslaught of this exceptionally dangerous terrorist movement, it will only grow stronger until it can target us on the streets of Britain."
Meanwhile, the Bishop of Leeds has said "many" senior clergy in the Church of England are seriously concerned about Britain's approach to the handling of the Iraq crisis.
The Right Rev Nicholas Baines has written to Mr Cameron asking about the government's overall strategy in response to the humanitarian situation and to IS.
"Behind this question is the serious concern that we do not seem to have a coherent or comprehensive approach to Islamist extremism as it is developing across the globe," he wrote, in a letter published on his website and. and backed by the Archbishop of Canterbury.
He criticised an "increasing silence" about the plight of tens of thousands of persecuted Christians in Iraq, and questioned whether they would be offered asylum in the UK.
In June, Mr Cameron claimed that fighters from the group were plotting attacks against the UK - although the threat level has remained unchanged.
At the time, he said up to 400 British nationals were fighting alongside militant groups in Syria.
He added that several people had already been stopped from travelling to conflict zones, and preparing a terrorism offence abroad would become a crime under legislation to be put before Parliament in the next few months.
Downing Street later revealed that the UK security service had arrested 65 people suspected of Syria-related jihadist activities during an 18-month period - including 40 in the first quarter of this year.
On 29 June, IS declared that it had created a caliphate, or Islamic state, stretching from Aleppo in Syria to the province of Diyala in Iraq.
Writing in the Sunday Telegraph, Mr Cameron warns that if IS is able to "carve out its so-called caliphate" the UK would be "facing a terrorist state on the shores of the Mediterranean and bordering a Nato member".
In the past few days, the UK military has made aid drops to hundreds of people stranded in northern Iraq while the US has launched air strikes to target the militant group.
But the prime minister writes that a "broader political, diplomatic and security response" is needed, in addition to such humanitarian action.
"Uncompromising action""First, we need a firm security response, whether that is military action to go after the terrorists, international co-operation on intelligence and counter-terrorism or uncompromising action against terrorists at home," he writes.
The prime minister suggests that anyone walking around in the UK with an Islamic state flag should be arrested.
Mr Cameron makes clear that he does not see this as a "war on terror" but "a battle between Islam on the one hand and extremists who want to abuse Islam on the other".
The extreme Sunni group, which overran Mosul this summer, has been accused of massacring non-Muslims.
Whole communities of Yazidis and Christians have been forced to flee in the north, along with Shia Iraqis, whom IS do not regard as true Muslims.
Meanwhile, Iraq's military response to a rapid advance by IS has been hampered by political turmoil in Baghdad.
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