Former head of the Royal Navy Lord West of Spithead warned there was a worrying lack of clarity about the UK's strategy and a danger of "mission creep".
General Sir Richard Dannatt, former head of the British Army, meanwhile insisted "the nation would expect" Parliament to be recalled to allow MPs and peers to debate Britain's growing involvement in the conflict and said it was unwise for the Government to publicly rule out British boots on the ground.
Iraqi security forces have launched an offensive to take back the city of Tikrit, which has been occupied by Islamic State (IS) since June, following Monday's victory in the retaking of the Mosul Dam from militants.
Britain is poised to provide weapons to Kurdish troops fighting the extremists of IS in northern Iraq as well as continued humanitarian aid.
First Minister Alex Salmond said the Scottish Government was extremely concerned about the humanitarian situation in northern Iraq and had written to the UK Government to offer assistance.
But he added: "In terms of any military intervention it is imperative that the UK Government moves through the United Nations route and does not repeat the tragic mistakes of 11 years ago. There can be no effective military action without international legality.
"The appalling consequences of previous blunders in this regard are what are visiting us now in Iraq and elsewhere."
As the debate over Britain's role continued to rage, the Prime Minister was photographed sipping coffee with wife Samantha at Surfside cafe on Polzeath beach in Wadebridge.
The couple appeared relaxed as they sat at a window table that overlooked a busy beach.
Mr Cameron has insisted he remains in control during the holiday and is "always within a few feet of a BlackBerry".
Sir Richard, who was a defence adviser to the PM after he retired as chief of the general staff in 2009, warned that MPs and Lords would get "very frustrated" if they did not get to address the situation before Parliament sits again. MPs are due to return briefly in September but peers will not have a chance to debate the issue until October.
Speaking from Cornwall, Sir Richard told the BBC: "I think there will come a point, as this general set of circumstances unfolds, when Parliament needs to come back together, people need to have a full debate about it and express their point of view.
"I think the nation would expect that."
Lord West, former First Sea Lord, told a newspaper: "We need to be very certain of our ultimate aim and we don't seem to have great clarity of vision about what we are going to do there. It is very worrying.
"There is always a danger of mission creep. We need to be very clear on our game plan - what, at the end, is going to make our nation and the globe more secure.
"The problem with getting involved is there is no way you want to become fully involved. It would be a dangerous step to have troops on the ground.
"But you look at what the Islamic State is doing and it is incumbent to do something. We should use limited military capabilities: reconnaissance capabilities, intelligence, possibly airstrikes and using Special Forces to mark targets for firing from drones."
Mr Cameron has pledged that the UK will not be "dragged into a war in Iraq" but Labour has criticised the Government's strategy, claiming the British role in the crisis is unclear.
Earlier this month, the PM cut short a trip to Portugal to respond to the emergency and has insisted he will do the same again "instantly" if necessary.
"Wherever I am, wherever I am in the world I am always within a few feet of a BlackBerry and an ability to manage things should they need to be managed," he said.
"And indeed, as I have done on I think almost every holiday that I have enjoyed over the last few years, to return instantly should that be necessary."