British pilots have launched at least 39 missile strikes against suspected Taliban insurgents from American drones based in Afghanistan, according to new figures.

The details have emerged from the Ministry of Defence, which has for the first time disclosed how RAF crews using unmanned US aircraft have launched missiles in conflict zones.

The MoD insists British drone pilots always operate under UK rules of engagement, whatever asset they are flying.

However, campaigners have called for increased scrutiny over the use of the aircraft and condemned a lack of transparency about the programmes run by the American and British armed forces. British crews piloted US Reaper and Predator drones in Afghanistan on 2,150 occasions between 2006 and 2012 – an average of almost once a day.

That does not include the thousands of missions British forces have flown with their own fleet of 10 Reaper aircraft.

The MoD has also for the first time provided a breakdown of the number of missiles fired by UK drones every month between May 2008 and April 2013.

These attacks peaked in November 2011, when 25 missiles were fired. There has been only one month in which no missiles were launched – December 2008.

Latest figures show RAF drones fired 94 Hellfire missiles in Afghanistan during 2013, bringing the total number of munitions and bombs fired by British unmanned systems since 2008 to 457.

The figures were released last week, nine months after a freedom of information (FoI) request, to Drone Wars UK, a website that researches and monitors the British use of unmanned technologies.

The website's creator, Chris Cole, said: "This latest revelation once again demonstrates the secrecy surrounding the use of armed drones and once again underlines the need for increased scrutiny and greater transparency about their use.

"The nature of drone technology means they are being used with little or no public accountability.

"Unless we act now to curb this new weaponry it seems inevitable that drones will increasingly be used to launch secret and unaccountable military attacks leading to global instability and increased insecurity."

Heather Barr, of Human Rights Watch, added: "One of the problems with drone programmes from the beginning has been a near-total lack of transparency about the scale of the programmes, where they were operating, what their goals are, and who they are targeting and how. If it is now not even possible to know which country has its finger on the button, that adds yet another layer of confusion which will make accountability even more elusive."

The MoD statement to Drone Wars UK makes clear British pilots were flying US drones in Afghanistan long before the RAF had any unmanned aircraft of its own; the UK did not start flying its own Reapers in Afghanistan until mid-2008.

The FoI reply states: "Between October 2006 and 31 December 2012, UK aircrew had flown approximately 2,150 operational missions using US Reaper and Predator remotely piloted air systems (RPAS) in support of operations in Afghanistan and Libya.

"UK and US personnel also operate both nations' RPAS as part of the launch and recovery phase in Afghanistan. Of the 2,150 missions flown by UK personnel, there were 271 missions in Afghanistan when UK personnel utilised a US Reaper as a UK Reaper was unavailable. During these missions, UK personnel released 39 weapons."

While the MoD gave some details about operations in which the UK has borrowed US equipment, it refused to give details about operations flown by British crews formally within the US air force. "I am withholding information about weapons released by UK personnel embedded with the US air force on operations in Afghanistan and Libya," the letter says.

"I have carried out a public interest test. Releasing the information would provide a limited additional understanding of the UK's relationship with the US over that which is already in the public domain in relation to operations in Afghanistan.

"Releasing the information concerned, when the other government has not given permission to do so, would undermine the relationship of trust that exists between the UK and other countries. Consequently, I am withholding the information. Information is not held for operations in Iraq."

Cole said it was essential that information about drone operations undertaken by British RAF pilots while embedded with US forces be made available for public scrutiny and debate."

He called on the defence select committee, which is holding an inquiry into drones, to reconsider its decision to hold its sessions in private.

The MoD insists drones operate under the same rules as other aircraft: "The RAF and USAF continue to work closely to provide persistent armed intelligence and surveillance to allied armed forces in Afghanistan. This occasionally involves the RAF utilising USAF assets when UK aircraft are unavailable. These aircraft always operate on UK tasks, with RAF aircrew in control, using UK rules of engagement."

The MoD has yet to decide what it will do with its fleet of armed Reapers once UK forces have left Helmand province at the end of combat operations this December.

Under current rules, the aircraft cannot be flown in the UK. The MoD is considering options that might include basing the drones in Kenya to support military operations against terrorists in the Horn of Africa.