The SAS is on the ground in Iraq as the United Nations warned that humanitarian crisis there had reached its highest level.
The Daily Telegraph reported that Baroness Nicholson of Winterbourne, the trade envoy to Iraq, had said British special forces had been working with US troops for "six weeks or more".
A Ministry of Defence spokesman said: "We do not comment on UK special forces."
The reported remarks on SAS involvement emerged after Mr Cameron said "detailed plans are now being put in place" for an international mission to rescue Yazidis trapped in the Mount Sinjar region and that Britain "will play a role in delivering it".
Asked about calls for the UK to follow France in arming Kurdish fighters, who are being outgunned by Isis forces, or to commit British fighter planes to air strikes, Mr Cameron said: "Of course we support the Kurds and we should continue to support the Kurds, and in terms of the ammunition they are getting, Britain is playing a role in helping to get that to them."
The PM returned from a family holiday to chair a meeting of the Government's Cobra emergency committee after which he insisted the UK involvement remained a humanitarian mission.
Afterwards, he dismissed demands for Parliament to be recalled, saying it was unnecessary at this stage but would be kept under review. "We need a plan to get these people off that mountain and get them to a place of safety," he said.
Mr Cameron is continuing to face calls to directly arm K urdish forces or join the US in air strikes against Islamic State (IS) extremist fighters.
Another 130 US troops have also arrived in Irbil on, what the Pentagon described as a temporary mission to assess the scope of the humanitarian crisis on Sinjar Mountain.
UN special representative Nickolay Mladenov said declaring Iraq a "level three emergency" will help trigger additional goods, funds and assets to respond to the needs of tens of thousands of people displaced by the IS extremist group's offensive.
He said Iraq has been raised to the most serious level "given the scale and complexity of the current humanitarian catastrophe".