David Cameron has indicated that he is sympathetic to calls for some of Britain's international aid budget to be switched to the Ministry of Defence.
The Prime Minister said that providing security in states torn apart by conflict was one of the "most fundamental parts" of international development.
His comments came amid reports that the MoD was pressing the Department for International Development (Dfid) to help foot the bill for flights on military aircraft, some naval patrols and body armour.
"There is an argument that one of the most fundamental parts of development is security. You don't get development without peace and security," Mr Cameron told BBC Radio 4's The World at One.
"The rules already allow some of your aid money to be spent on security issues. We should make sure that, within the rules, we are doing what is necessary - not to make the numbers add up, but actually so that development takes place.
"If you look at the most fragile, conflict-affected states, they haven't met one of the Millennium Development Goals between them.
"There are rules about this, but within the rules should we be asking are we actually helping countries that are desperately poor and affected by conflict - are we helping them to develop in the right way? I think we should ask that question."
Ministers are currently locked in a battle for resources ahead of next month's spending review in which those departments whose budgets are not ring-fenced are expected to face further cuts.
Mr Cameron is committed to maintain Britain's spending on overseas aid, but allowing some of the money to go to defence could be a way of helping the hard-pressed armed forces which have already seen big cuts.
The MoD confirmed that it was looking at the issue with Dfid "Improving stability and security in fragile and conflict-affected states is vital to our national security - that's why we're working with Dfid to look at how best we can use our resources overseas ahead of a discussion at the National Security Council before the summer," a spokesman said.