PM working on plan to be agreed at Nato summit with view that any response has to be coordinated international effort

David Cameron is planning to indicate to Barack Obama and other Nato leaders at a summit in Wales next week that Britain is keeping open the option of joining the US in launching air strikes against forces of the Islamic State (Isis) in Iraq.

As Washington seeks to build a multi-national coalition to support an expansion of air strikes against the jihadists, which could extend to targets in Syria, the prime minister returned to work in Downing Street to intensify plans for an emergency action plan to be agreed at the Nato summit. The plan will focus on new threats posed by Isis in Iraq and Syria and by the conflict between Russia and Ukraine whose president, Petro Poroshenko, will attend the Nato summit.

Alan Duncan, former international development minister and the PM's special envoy to Oman, raised the prospect of Britain eventually joining air strikes against Isis as long as they took place within an international coalition. Duncan said: "This is such a complicated conflict that any response needs to be a well co-ordinated international effort."

Obama announced this week that the US was seeking to build a coalition to "take the fight to these barbaric terrorists" amid growing fears over the plight of members of Iraq's Turkomans minority in the northern town of Amerli. The New York Times reported yesterday that the White House expects that Britain and Australia will join the US in an air campaign. Australia left the door open to contributing to US-led air strikes in Iraq but indicated it was not considering putting combat forces on the ground. The office of the Australian prime minister Tony Abbott said he would consider any request from the US "based on whether there is an achievable overall humanitarian purpose and a clear and proportionate role for Australia, as well as on a careful assessment of the risks".

Downing Street said that Britain has received no request from Washington for the UK to contribute to air strikes. A senior government source said: "There would need to be a lot of discussion before Britain got involved in air strikes and we have not had any specific requests from the US on this. Our focus has not been on air strikes. It hasn't been on the table, and it has not been discussed."

But Britain has crafted a careful position in its response to the threat posed by Isis that leaves open the eventual possibility of joining air strikes.

The prime minister, who said earlier this month that Britain should be prepared to deploy its military "prowess" to tackle Isis, has been explicit in ruling out deploying ground troops.

But the foreign secretary, Philip Hammond, adopted a more cautious approach on air strikes, saying simply on 18 August that there are no plans for Britain to join the US in launching air strikes.

The formulation by ministers is not designed to signal that Britain is preparing to take part in air strikes, not least because the White House has not requested any British involvement. But ministers want to leave some room for manoeuvre if Obama seeks to build consensus for air strikes in the US by requesting help from close allies such as Britain.

British officials are acutely conscious of Obama's landmark speech to the US Military Academy at West Point, New York, in May in which the president made clear that the US can no longer act as the world's policeman. Obama said in his speech that the US "must always lead on the world stage" before adding: "US military action cannot be the only – or even primary – component of our leadership in every instance."

The military "prowess" cited by the PM has so far involved joining the US in dropping humanitarian supplies to Yazidi refugees on Mount Sinjar. RAF Tornado jets, which initially carried out surveillance flights to identify the needs of refugees, expanded their mission into northern Iraq to gather intelligence on Isis.

It is understood that the current discussions are focusing on whether to mount a similar humanitarian mission to help the Turkoman minority in the northern town of Amerli.

Officials have been aware of the dangers in Amerli for some time. But attention moved away from Amerli after Isis released the video featuring the beheading of the US journalist James Foley.

The possible moves towards expanded air strikes came as Isis and the Syrian government, which is engaged in bitter fighting with the jihadis, were both accused by UN investigators of committing war crimes and crimes against humanity. The UN commission of inquiry on Syria accused Damascus of dropping barrel bombs on civilian areas, including some which were believed to contain chlorine on eight occasions in April.

The report said: "Violence has bled over the borders of the Syrian Arab republic, with extremism fuelling the conflict's heightened brutality."

The report also accused Isis forces in northern Syria of waging a campaign to instil fear, including amputations, public execution-style killings and whippings, according to the Associated Press.

"In areas of Syria under [Isis] control, particularly in the north and north-east of the country, Fridays are regularly marked by executions, amputations and lashings in public squares," the independent commission of inquiry on the human rights situation in Syria said. "Executions in public spaces have become a common spectacle on Fridays in [Isis power-base] Raqqa and in Isis-controlled areas of Aleppo governorate. Bodies of those killed are placed on display for several days, terrorising the local population."