Britain's Cameron vows to ‘hunt down’ ISIL after citizen executed

Haine's murder was the third beheading of a Western hostage in less than a month. American journalists James Foley and Steven Sotloff were killed by the group in a similar manner.

The latest video, which Britain’s Foreign Office verified as genuine, featured a fourth hostage – also British – who was threatened with execution if Britain does not comply with Islamic State’s demands.

The video’s masked perpetrator, who spoke in an English accent, said Islamic State would continue to target British hostages so long as Britain cooperated with the United States in its fight against the group.

The United States resumed airstrikes in Iraq in August for the first time since the withdrawal of the final U.S. troops from the country in 2011.

The raids followed major gains by Islamic State, which has declared an Islamic caliphate in areas it controls in Syria and Iraq.

Obama is now calling for a coalition of Western and Middle Eastern countries to fight Islamic State and has said the U.S. intends to bomb Islamic State positions in Syria.

Britain has delivered humanitarian aid, carried out surveillance, given weapons to Kurds fighting Islamic State and promised training in Iraq. On military action, Britain has only ruled out deploying combat troops on the ground.

Cameron on Sunday said Britain would continue to battle Islamic State on multiple fronts, but for now refrain from participating in airstrikes.

"As this strategy intensifies we are ready to take whatever steps are necessary to deal with this threat and keep our country safe," he said in a TV statement from his office.

"Step by step, we must drive back, dismantle, and ultimately destroy ISIL [Islamic State] and what it stands for. We will do so in a calm deliberate way, but with an iron determination."

Britain has in the past often been the first country to join U.S. military action overseas, but war-weary public opinion, parliament's rejection last year of air strikes on Syria, and sensitivities surrounding Scotland's independence referendum on Thursday mean Cameron is reticent this time round.

President Barack Obama also failed to win support for airstrikes in war-torn Syria last year after government forces reportedly used chemical weapons against civilians. However, following the murders of Foley and Sotloff, public support for military action has spiked.

Obama said in a statement on Wednesday that the U.S. would lead a “broad coalition” of nations from the region and around the world to “degrade and ultimately destroy” Islamic State. However, promised that efforts “will not involve American combat troops fighting on foreign soil.”

U.S. intervention, Obama said, would include airstrikes, supplied intelligence, weapons, logistics and training to “partner forces on the ground.”

A Syrian government minister said on Thursday that any military action on its soil would need prior approval, or be viewed as an act of aggression. Syria has said it would assist any coalition in fighting Islamic State — a partnership the U.S. aims to avoid.

Australian Prime Minister Tony Abbott said on Sunday that a 600-strong force comprising some 400 air force personnel and 200 special forces soldiers would be deployed to a U.S. military base in the United Arab Emirates to assist in the fight against Islamic State.

A number of countries have responded to Obama's call to join an international coalition against the group, but Australia is the first to publicly provide specific troop numbers and military hardware for the mission.

Al Jazeera and wire services