Cameron on ISIS: 'They are Not Muslims. They are Monsters.'

Not 24 hours after the brutal video showing the beheading of humanitarian aid worker David Haines was released, the British government said it would seek justice while stressing that ISIS doesn't represent Islam.

The killer shown in all three ISIS beheading videos -- Haines, James Foley, Steve Sotloff -- speaks with a London accent.

"David Haines was a British hero. The fact that an aid worker was taken, held and brutally murdered at the hands of ISIL sums up what this organisation stands for," British Prime Minister David Cameron said in a statement today. Haines, a former Royal Air Force engineer, was kidnapped 18 months ago, and the prime minister said "the whole country, like his grieving family, can be incredibly proud of what he did and what he stood for in his humanitarian mission."

"They are killing and slaughtering thousands of people, Muslims, Christians, minorities across Iraq and Syria," Cameron said of ISIS. "They boast of their brutality. They claim to do this in the name of Islam. That is nonsense. Islam is a religion of peace."

"They are not Muslims. They are monsters. They make no secret of their desire to do as much harm not just in the Middle East, but to any countries or peoples who seek to stand in their way or dare to stand for values they disagree with."

Cameron said "the British people need to know that this is a fanatical organisation called ISIL, that has not only murdered a British hostage: They have planned - and continue to plan - attacks across Europe and in our country."

He said they will confront the terrorist state "in a calm, deliberate way - but with an iron determination" with allies.

"The United States is taking direct military action. We support their efforts," Cameron continued. "British Tornadoes and surveillance aircraft have been helping with intelligence gathering and logistics. This is not about British combat troops on the ground. It is about working with others to extinguish this terrorist threat. As this strategy intensifies, we are ready to take whatever steps are necessary to deal with this threat and keep our country safe."

"...It falls to the Government, and to each and every one of us, to drain this poison from our society and to take on this warped ideology that is radicalising some of our young people."

Deputy Prime Minister Nick Clegg issued a shorter statement calling Haines' beheading "a crime of the most horrendous kind."

“This murderous organisation calls itself Islamic State," Clegg said. “But it is not a state, it is a brutal terrorist outfit whose actions are an affront to every peace loving Muslim around the world. No religion could possibly justify such grotesque acts."

The Islamic Society of Britain sent a letter to Cameron vowing that they "shall take every opportunity to continue to say clearly and loudly 'not in our name and not for our faith'."

"We do not believe the terror group responsible should be given the credence and standing they seek by styling themselves ‘Islamic State’. It is neither Islamic, nor is it a State. The group has no standing with faithful Muslims, nor among the international community of nations. It clearly will never accept the obligations that any legitimate state has, including the responsibility to protect citizens and uphold human rights," the Islamic Society wrote.

"So we believe the media, civic society and governments should refuse to legitimise these ludicrous Caliphate fantasies by accepting or propagating this name."

They proposed that "Un-Islamic State" could be an "accurate and fair alternative name to describe this group and its agenda – and we will begin to call it that."

"We are sure that most British Muslims would agree that 'Un-Islamic State' is a considerably more fitting label for this poisonous group – and hope that our fellow citizens will join us in that," wrote the organization. "We know that this would be one small, symbolic step and that we must all work together to build the inclusion and integration in British society that would repel these poisonous ideas. But we believe that it would help and look forward to your response."