WASHINGTON/LONDON - 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 programme Face The Nation.
Offers of Arab air participation have been made both to US 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, US 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, US officials said, justifying them largely on the basis of defending Iraq from militants who have taken shelter in neighbouring 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 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 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.