Several Arab states, believed to include the United Arab Emirates and Saudi Arabia, have offered to help attack Islamic State (Isis) targets in Syria and Iraq, in a major boost for US efforts to build a broad coalition against the Sunni insurgent group.

The offers, reported by senior western diplomats, came in the wake of widespread international condemnation of the murder of the British hostage, David Haines, and a pledge by Australia to help the military effort. John Kerry, the US secretary of state, said on Sunday he was "extremely encouraged" by pledges made so far. Kerry is to join Iraqi, Arab and other western ministers at a conference in Paris on Monday to agree ways to support the new Baghdad government in the war against the jihadi group. Arab participation in military action would help give a wider sense of legitimacy to the campaign.

US officials declined to say which countries had offered help, but one appeared to be the UAE, whose aircraft recently bombed Islamist militia targets in Libya from bases in Egypt.

A senior western source told the Guardian that Saudi Arabia felt so threatened by Isis that it was prepared to act in a frontline role. "There is a very real possibility that we could have the Saudi air force bombing targets inside Syria. That is a remarkable development, and something the US would be very pleased to see."

Another senior official said that Saudi Arabia was now far more willing to play an open role in the campaign against Isis than during the 1991 Gulf war and the 2003 invasion of Iraq. In both previous campaigns, Riyadh allowed its military bases to be used by US forces, but did not commit its own troops or airmen.

This time, Riyadh sees Isis as a direct threat to Saudi Arabia. "They actually see themselves as the real target. "They know that they have to step up, and they are ready to, from what we can see," the official said.

A US official told the New York Times that the US Central Command, which oversees military operations in the Middle East, had received offers from several Arab states. "There have been offers both to Centcom and to the Iraqis of Arab countries taking more aggressive kinetic action."

France has indicated that it will back US air strikes against Isis after its president, François Hollande, expressed support for the Iraqi prime minister, Haider al-Abadi, and moves to undercut Sunni support for Isis. But Turkey, which borders on both Iraq and Syria, has quietly made clear that it would not take part or allow its bases to be used for combat operations – a disappointment coming from Nato's only Muslim member.

Isis fighters during a parade in Raqqa, Syria. Photograph: AP

Syrian ministers have repeated calls for Damascus to join the coalition, though the US and Britain – backed by their Gulf allies – have insisted president Bashar al-Assad cannot take part because he has "lost all legitimacy" in the course of a war that has cost 200,000 lives.

Kerry said the US would not coordinate any attacks with Syria, but added in an interview on CBS's Face the Nation: "We will certainly want to deconflict and make certain that they're (Syria) not about to do something that they might regret even more seriously."

Britain will be represented at the Paris talks by Philip Hammond, the foreign secretary, with David Cameron facing conflicting pressures over whether to participate in air strikes or restrict the country to delivering humanitarian aid, surveillance, and arming and training Kurdish and Iraqi forces. Cameron made clear on Sunday that he supports US strikes and "whatever steps are necessary" while keeping options open. The Haines murder may change the dynamic of the arguments.

Details of how the anti-Isis campaign will be waged are still sketchy, though the US reportedly discussed basing and overflight rights at talks in Jeddah last week with the Saudis and the other Gulf states as well as Iraq, Lebanon, Jordan and Egypt. All expressed support for a "coordinated military campaign".

"I can tell you right here and now that 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.

Officials familiar with high level discussions between Riyadh and Washington say both sides are determined to avoid the perception in the Sunni world that the upcoming campaign will benefit Iran and its Shia and Alawite proxies. "The Saudis are the power base of the Sunni world and it is time for them to provide an alternative to Isis," said a regional official. "They know what is expected of them and this time you will see them acting directly."

Denis McDonough, the White House chief of staff, said that while air strikes would weaken Isis, "it's going to be Iraqi and other boots on the ground" that would the key to defeating the terrorists.

"To destroy Isil[Isis] we need to have a force, an anvil against which they will be pushed," McDonough said on CNN's State of the Union.

"It will be a coalition that includes not only our friends in Europe and Asia but also our partners in the region, Muslim states, Sunni states. We're going to use our unique capabilities, air power, ISR [intelligence, surveillance and reconnaissance] and our training ability to make sure Iraqi forces on one side and Syrian opposition forces on the other side of the border can take the fight to Isil."

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Obama's condemnation of Haines's murder was echoed in other capitals, with the German foreign minister, Frank-Walter Steinmeier, calling the killing "an abhorrent act of barbaric violence beyond all limits of human civilisation."