Nicola Sturgeon has cast doubt on the UK Government’s estimate of the number of moderate fighters on the ground in Syria.
The first minister characterised the 70,000 figure revealed by David Cameron in the Commons this week as a “heroic assumption”.
She praised the prime minister’s tone as he made his case for extending British military intervention.
But she confirmed the SNP’s position not to back air strikes against Islamic State in Syria unless questions on ground troops and reconstruction plans are answered.
Ms Sturgeon said Britain could not get into the “realms of doing anything just in the interests of doing something”.
Speaking after the British Irish Council summit in London, she said: “Who is it on the ground that recaptures and governs territory that Isil hopefully is pushed out of?
“The numbers the prime minister was citing in the House of Commons I think perhaps fell into the category of heroic assumptions.
“Many of these moderate Syrian fighters are of course under attack by Russian forces.
“It’s firstly important in terms of directing air strikes to minimise civilian casualties and then in taking the territory.
“That’s not just a minor point – that is a critical point in terms of the effectiveness of air strikes.
“The second point, which is longer-term but still important, is the issue of reconstruction.
“That is where there is a significant lesson to be learned from Iraq.”
Ms Sturgeon insisted the SNP’s stance was not a “knee-jerk” reaction or ideologically driven.
“As things stand we are not persuaded and we will not vote for air strikes,” she said.
“But we will continue to listen up to the point at which the vote happens, if it does.”
The issue is expected to be put to the House towards the middle of next week.
Labour leader Jeremy Corbyn has written to his party’s MPs saying he could not support the current proposal on the grounds he did not believe it would protect Britain’s security.
But Mr Cameron has insisted all the advice he has received indicates the risk of inaction is greater than that attached to not not intervening.
His spokeswoman said the intelligence about 70,000 troops was based on “detailed analysis”.