The UK has focused on targeting IS infrastructure such as oil fields
British airstrikes in Syria have killed or injured just seven Islamic State militants, according to estimates from the country's defence ministry.
Four attacks have resulted in IS casualties since David Cameron won parliamentary support to extend airstrikes in December, the figures have revealed.
The prime minister had cited the high-precision Brimstone missile as a UK asset that could make a "meaningful difference" to the international efforts in Syria.
But the figures - accurate to the end of January - show none of the RAF strikes have so far used the Brimstone.
The UK has focused on targeting IS infrastructure such as oil fields which provide revenue for the fighters.
A UK defence ministry spokesman said: "We are playing a crucial role in a campaign that will take time and patience. Using the right weapon for each scenario, RAF jets have struck Daesh (IS) almost 600 times.
"In Iraq we have helped to drive them out of Sinjar and Ramadi. In Syria, we have severely weakened them by targeting their key infrastructure."
On Christmas Day, one militant was killed or injured at a checkpoint south of Raqqa, IS' de facto capital in Syria, after an RAF Reaper drone fired a Hellfire missile.
Two more fighters from IS were hit on the same day in Tabqa, also with a Hellfire.
On 11 January, two fighters were killed or injured in a Hellfire strike in Al Busayrah, the figures showed, while two more were hit by a Paveway IV guided bomb on 15 January.
The figures were released following a Freedom of Information request by the Huffington Post.