Leaders and diplomats from more than 20 countries have gathered for a crisis meeting in Paris as the prospect of Britain joining military action against the Islamic State (Isis) jihadi group moves closer.

Opening the conference at the French foreign ministry, President François Hollande urged western and Arab countries to engage "clearly, loyally and strongly at the side of the Iraqi authorities". There was "no time to lose" in dealing with the threat from Isis. "Iraq's combat against terrorism is also ours," he said.

Monday's gathering came just a day after Isis released a video showing a hooded jihadi killing the British aid worker David Haines, 44, the third western hostage to be decapitated by the group in a month.

Russia's foreign minister, Sergei Lavrov, and his US and UK counterparts, John Kerry and Philip Hammond, were at the meeting.

On Sunday, the British prime minister, David Cameron, led international condemnation of the ritualised killing of Haines and threats against a second UK citizen, Alan Henning.

As western diplomats reported that several Arab states had offered to join a US-led coalition carrying out air strikes on the insurgents, Cameron said the UK was "ready to take whatever steps are necessary" to deal with the threat of Isis.

Before the conference began, France's foreign minister, Laurent Fabius, announced that French aircraft were to begin reconnaissance flights over Iraq. "We told the Iraqis we were available and asked them for authorisation," he told Europe 1, but added there was "no question of sending ground troops".

The Iraqi president, Fouad Masoum, who has appealed for rapid air intervention, told those gathered in Paris that Daesh, the Arabic acronym for Isis, was open in its aims and called for concerted action to defeat what he described as terrorists and criminals, who represented a "new form of terrorism".

"Daesh leaders are working at setting up a state and do not deny the existence of terrorist volunteers from European countries who may or may not have dual nationality … it goes beyond what we experienced before with al-Qaida," he said. "These criminals are experts at the brainwashing of young people in these regions. They control and prepare them for terrorist actions. They use modern technology to spread propaganda and terror through the internet and social media networks.

"Iraq is here today to show its will to stand up against this terrorism enemy that makes no distinction between Iraqis. We must sent a strong message to the orphaned mothers who have lost their children that we stand beside our people."

More than 40 countries have signed up to a US-led plan, but not all are willing to offer Iraq direct military support. Arab participation in military action would give a wider sense of legitimacy to the campaign. No Arab state has publicly promised to participate in military action but it is believed several have in private, including Saudi Arabia and the United Arab Emirates, the latter of which 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 Saudi Arabia was far more willing to play an open role in the offensive against Isis than it did during the 1991 Gulf war and the 2003 invasion of Iraq. On those campaigns, Riyadh allowed its military bases to be used by US forces, but did not commit its service personnel. This time, Riyadh sees Isis as a direct threat.

"They actually see themselves as the real target," the official said. "They know that they have to step up, and they are ready to, from what we can see."

Australia has promised to send 600 troops and eight fighter jets to help the US fight what the prime minister, Tony Abbott, called the Isis "death cult".

The UK and the US have ruled out sending ground troops to areas of Iraq and Syria controlled by Isis jihadis, but Cameron hinted that the UK would be prepared to join the US in conducting a bombing campaign against Isis as part of a wider alliance. Cameron heightened his rhetoric against the "monsters" in Isis after a video emerged on Saturday night purporting to show a masked fighter beheading Haines before threatening to kill Henning, a 47-year-old former taxi driver and volunteer on an aid convoy.

The jihadi, who had the same London accent as the man in previous hostage videos, warned Cameron that he would have more blood on his hands if the UK continued to fight Isis. It is the third recent film; the others showed the murder of two US journalists – James Foley and Steven Sotloff – which have emerged at intervals of 10-14 days since 19 August.

In his statement, Cameron made no mention of recalling parliament to debate the possibility of air strikes, which he has promised to do before ordering military action unless confronted by an emergency. MPs are in recess for party conferences until 14 October.

He is under pressure from hawks within the Conservative party, including the former defence secretary Liam Fox, who are pushing for the UK to join air strikes. British military chiefs, including Lord Dannatt, a former head of the army, warned that the strength of Isis could grow if the government did not "confront and destroy" its influence.

The US carried out at least nine air strikes last week, while the UK has pledged to help arm the Kurdish resistance forces in Iraq. These actions were mentioned in the Isis video, entitled A Message to the Allies of America, as reasons for the killing of Haines.

The US president, Barack Obama, condemned the "barbaric murder" of Haines. "Our hearts go out to the family of Mr Haines and to the people of the United Kingdom," he said. "The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve. We will work with the UK and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world."