Washington said countries in the Middle East had offered to join air strikes against Islamic State militants and Australia said it would send troops, but Britain held back even after the group beheaded a British hostage and threatened to kill another.
US Secretary of State John Kerry has been touring the Middle East to try to secure backing for US efforts to build a coalition to fight the Islamic State militants who have grabbed territory in Syria and Iraq.
"We have countries in this region, countries outside of this region, in addition to the United States, all of whom are prepared to engage in military assistance, in actual strikes if that is what it requires," Mr Kerry said.
"And we also have a growing number of people who are prepared to do all the other things," he said in remarks broadcast on the CBS programme Face the Nation.
Britain would be "mad" not to deploy its military prowess to tackle Islamic State, London Mayor Boris Johnson said. Writing in the Daily Telegraph, Mr Johnson said:
For all our occasional spasms of self-doubt, we are one of the great powers of the world with some of the finest armed forces. We would be mad not to use our defence capability, where we can, to make the world a better place.
The brutal murder of one British aid worker and threat to kill a second will add urgency to efforts today to form a substantial international coalition to mount an assault on Islamic State (IS) extremists.
Foreign Secretary Philip Hammond will attend a summit in Paris as Washington seeks military commitments from more regional and global allies for the fight to push back the jihadist advance.
Today's emergency meeting of the Government's Cobra committee did not produce a decision to respond militarily to David Haines' death.
The Government said a knee-jerk reaction would be allowing the militants to set the agenda.
Instead, they will continue the long-term strategy of destroying Islamic State (IS) as part of a coalition.
ITV News Political Correspondent Carl Dinnen reports:
Former Defence Secretary Liam Fox claims it is "inevitable" that the UK will take military action against Islamic State (IS) terrorists in Iraq.
Speaking on BBC Radio 5 Live, Dr Fox argued that IS, also known as Isis or Isil, could not be overcome without outside help.
"It's inevitable we will take military action against Isis because our allies on the ground, the Kurds and the Iraqis simply don't have the level of the air power that they will require to degrade Isis."
David Cameron has issued a strongly-worded response to the apparent murder of aid worker David Haines, calling Islamic State terrorists "the embodiment of evil"
The Prime Minister also stressed that the group did not in any way represent Muslims, saying: "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."
He paid tribute to Mr Haines' family, who he said had born their ordeal with "extraordinary courage" and vowed that Britain would "hunt down those responsible and bring them to justice".
David Cameron has vowed to "extinguish" the threat of Islamic State terrorism.
The Prime Minister was speaking after a video was released that appears to show the murder of British aid worker David Haines.
He said the killing of Mr Haines "will not lead Britain to shirk our responsibility with our allies to deal with the threat this organisation poses".
He also warned that the terror group continued to plan attacks in Europe, saying: "They have planned and continue to plan attacks across Europe and in our country - we are a peaceful people, we do not seek out confrontation, but we need to understand, we cannot ignore this threat to our security and that of our allies."
US Secretary of State John Kerry has arrived in Paris following a tour of the Middle East region to gain support for an international coalition to combat jihadist groups.
Mr Kerry arrived at Charles De Gaulle Airport in Paris after flying from Cairo, where he met Egyptian President Abdel-Fattah el-Sissi and Foreign Minister Sameh Shurki.
Diplomatic talks are to be held in Paris on Monday to discuss helping Iraq fight off the Islamic State group.
Nearly 40 nations have agreed to contribute to what Mr Kerry said would be a worldwide fight to defeat the Islamic State militants in Iraq and Syria.
Signatories to a letter to David Cameron from a coalition of organisations and imams admit that UK Muslims need to do more to dissuade their young men from being misled into taking part in the extremist group rampaging through Syria and Iraq's "hatred and poison".
Sughra Ahmed, president of the Islamic Society of Britain, told the Observer:
These extremists aren't us. This isn't the Islam that we recognise.
But we need to do more than just say 'not in our name, not for our faith'. We need to work together and make sure that these fanatics don't get the propaganda that they feed off.
Australia will deploy military forces and fighter jets to the United Arab Emirates to assist the international coalition in fighting Islamic State militants, Prime Minister Tony Abbott has said.
In response to a formal request from the United States, Australia will deploy a 600-strong force in the coming days in preparation for military action against Islamic State.
Abbott said the deployment was a "prudent and proportionate" contribution to the international coalition and that his government had not yet made the decision to commit troops to combat action.