ISIL militants could grow strong enough to target people on the streets of Britain unless action is taken, British prime minister David Cameron has warned.

ISIL militants could grow strong enough to target people on the streets of Britain unless action is taken, British prime minister David Cameron has warned.

Cameron, writing in the Sunday Telegraph, said a "humanitarian response" to ISIL was not enough and a "firm security response" was needed.

"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."

He warned that if ISIL was 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".

"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 wrote.

David Cameron suggested that anyone walking around in the UK with an ISIL flag should be arrested.

Denying the possibility of sending troops to Iraq to fight ISIL terrorists, Cameron pointed out that Britain may cooperate with Iran to eradicate the terrorist threat posed by ISIL.

The British prime minister asserted that the West must cooperate with the Middle East countries including Saudi Arabia, Qatar, Egypt, Turkey and Iran to encounter the extremist group.