Speaking to BBC Scotland News, Swinney said he was "very worried" by security fears highlighted this week by the head of MI5, who named Russia as the primary threat. The SNP leader identified waters north of Shetland as an area of significant Russian activity and argued that the UK government did not have a sufficient presence there.
"I don't think we've got nearly the presence in those areas that we require today," he said. "There's an obligation on the United Kingdom government to make sure that the country is safe. And I don't think the defence posture of the United Kingdom enables me to have that confidence."
Swinney also denied that Scottish independence would weaken the country's security, saying it would give Scotland "more choices". He pointed to Canada's recent agreement to join the EU's SAFE defence programme as an example of co-operation. Scotland hosts the UK's nuclear submarine fleet at Faslane, which the SNP wants removed, but the party supports an independent Scotland joining Nato.
Prime Minister Andy Burnham has said his government is committed to investing in defence, including nuclear deterrence, while not seeking to escalate tensions with Russia. A Ministry of Defence spokesperson responded: "The first duty of government is to keep its citizens safe, and this government is investing £298bn over the next four years to ensure we can tackle rising threats. The Navy is helping to patrol Nato's northern flank, with this year's carrier strike group..."
The first minister's remarks come as Burnham is due to visit Scotland during the SNP conference, where security and defence are expected to be prominent themes.