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.
The United States resumed air strikes in Iraq in August for the first time since the 2011 withdrawal of the last US troops, fearful the militants would break the country up and use it as a base for attacks on the West.
The addition of Arab fighter jets would greatly strengthen the credibility of what is a risky and complicated campaign.
"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," 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 Sunday on the CBS program "Face the Nation".
Australia became the first country to detail troop numbers and aircraft to fight the militants in Iraq. It said it would send a 600 strong force and eight fighter jets to the region but did not intend to operate in Syria.
Russia, at odds with the West over Ukraine, has said any air strikes in Syria would be an act of aggression without the consent of President Bashar al-Assad or an international mandate.
Britain has often been the first country to join US military action overseas and is under pressure to get much tougher with IS after video footage of the killing of Briton David Haines by IS militants was released on Saturday.
In footage consistent with the filmed executions of two American journalists, James Foley and Steven Sotloff, in the past month, they also threatened to kill another British hostage.
Speaking after chairing a meeting of the government's emergency response committee in London, Prime Minister David Cameron called the killing of Haines, a 44 year-old Scottish aid worker, callous and brutal and hailed him as a "British hero".
"We will hunt down those responsible and bring them to justice no matter how long it takes," he said, calling IS "the embodiment of evil" and saying his government was prepared "to take whatever steps are necessary" against the militants.
But he did not announce any air strikes, mindful of war-weary public opinion, parliament's rejection last year of air strikes on Syria, and sensitivities surrounding Scotland's independence referendum on Thursday.
US allies are sceptical of how far Washington will commit to a conflict in which nearly every country in the region has a stake, set against the backdrop of Islam's 1,300-year-old rift between Sunnis and Shias.
On the CNN program "State of the Union," White House Chief of Staff Denis McDonough was asked if this coalition would need ground troops beyond opposition forces in Syria and Kurdish and government forces in Iraq.
"Ultimately to destroy ISIL we do need to have a force, an anvil against which they will be pushed - ideally Sunni forces," he said, using an acronym for Islamic State.
France has offered to take part in air strikes in Iraq and is expected to give more details this week on what it is willing to do, although its financial resources and forces are already stretched with more than 5,000 soldiers in West Africa.