UK air strikes have killed around 330 Islamic State (IS) fighters in the past year, the Defence Secretary said.
The number is "highly approximate", Michael Fallon said, and covers the period from September last year until the end of last month.
British extremists Reyaad Khan and Ruhul Amin were killed by a drone strike in the IS stronghold of Raqqah on August 21.
Downing Street said the decision to authorise the use of remote RAF aircraft to strike individuals in Syria plotting attacks on the UK was taken "some months ago".
A meeting of senior members of the National Security Council, chaired by David Cameron earlier this year, received advice from the Attorney General that drone attacks would be legal on grounds of self-defence.
Mr Fallon, answering a written question from Green MP Caroline Lucas, said the Government does not believe any civilians have been killed or injured by the strikes against IS, also known as ISIL.
He wrote: "The estimated number of ISIL fighters killed as a result of UK strikes from September 2014 to 31 August 2015 is around 330.
"This figure is highly approximate, not least given the absence of UK ground troops in a position to observe the effects of strike activity.
"We do not believe there have been any civilian casualties as a result of UK strike activity. Any overall estimates would be a matter for the coalition."
Labour leader Jeremy Corbyn has said drone attacks are "legally questionable".
He called for a "rapid political development" as a way to end the violence in Syria, where forces loyal to president Bashar Assad, rebels opposed to his rule and IS extremists have torn the country apart.
Mr Corbyn, chairman of the Stop the War coalition, is expected to oppose any proposal to expand the air campaign against IS.
The Prime Minister has indicated he could be prepared to seek Parliament's approval to extend the bombing campaign against IS from Iraq into Syria.