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 US 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."

Offers of Arab air participation have been made both to U.S. Central Command overseeing the American air campaign and to the Iraqi government, a senior State Department official said.

The official said the offers were not limited to air strikes on Iraq. "Some have indicated for quite a while a willingness to do them elsewhere," the official said. "We have to sort through all of that because you can't just go and bomb something."

As of Saturday, U.S. fighter jets had conducted 160 air strikes on Islamic State positions in Iraq. The United States will present a legal case before expanding them into Syria, U.S. officials said, justifying them largely on the basis of defending Iraq from militants who have taken shelter in neighboring Syria during its three-year civil war.

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 U.S. military action overseas and is under pressure to get much tougher with IS after video footage of the killing of Briton David Haines by the 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.

SUNNI 'ANVIL'

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.

U.S. allies are skeptical 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 Shi'ites.

Many fear there is not enough emphasis on ensuring the Iraqi government is strong and united enough to overcome sectarian divisions and run the country effectively after any intervention.

Britain and the United States have ruled out sending ground troops back into Iraq and Kerry did not say which countries had offered.

"We're not looking to put troops on the ground," he said. "There are some who have offered to do so, but we are not looking for that at this moment anyway."

On the CNN program "State of the Union," White House Chief of Staff Denis McDonough was asked if the 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.

'EXTREMELY ENCOURAGED'

On Thursday, Kerry won the backing for a "coordinated military campaign" from 10 Arab countries - Egypt, Iraq, Jordan, Lebanon and six Gulf states including rich rivals Saudi Arabia and Qatar.

"This is a strategy coming together as the coalition comes together and the countries declare what they are prepared to do," Kerry said in the interview, taped on Saturday in Egypt.

"I've been extremely encouraged to hear from all of the people that I've been meeting with about their readiness and willingness to participate," Kerry added.

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.

Michael McCaul, a Republican who chairs the House of Representatives Committee on Homeland Security, told the same CBS program that Prince Ali bin Al Hussein of Jordan told him "he is ready to put his troops into Syria to fight ISIS".

Washington could also try to persuade Egypt to put troops in Syria," McCaul said.

John Kerry will meet British Foreign Secretary Philip Hammond during a conference on Iraq in Paris on Monday. The conference brings Iraqi authorities together with about 30 countries and organizations to coordinate their response to Islamic State.

"It will also be the first time to really gauge what Russia thinks and is ready to do," a French diplomat said.

The diplomat said Syria was a different case.

"The situation is not the same either legally or militarily. We do not want to strengthen Assad, so we have to be sure that strikes there don't do that," the diplomat said. "We are ready to help Iraq's government, which has asked for our help, but not Assad's dictatorship." – Reuters