An MQ-9 Reaper drone takes off from Kandahar airbase in southern Afghanistan. Photograph: James Lee Harper/AFP/Getty Images

British forces are relying increasingly on unmanned drones to attack targets in Afghanistan, mirroring controversial tactics used by the US.

New Ministry of Defence figures show the RAF has fired 84 missiles from Reaper drones since they were first deployed there in June 2008, with more than 20 being fired over the past two months.

The RAF has not disclosed the number of US-made Reapers deployed in Afghanistan, but say they will double the total over the next two years. Defence chiefs say they have been slow to recognise their potential, both in a surveillance role and as a weapons carrier.

They are launched from a base in Kandahar, but are controlled remotely thousands of miles away by a squadron of some 90 RAF personnel based at Creech US air force base in Nevada.

The drones can carry out surveillance – what the RAF describe as a "staring eye" – of the battlefield around the clock, far longer than conventional manned aircraft. They are highly suitable in Afghanistan where they are not generally vulnerable to enemy fire, defence officials say.

Once a target has been identified, the RAF remote controllers can instruct the drones to fire their two 500lb laser-guided bombs and four Hellfire missiles.

The MoD does not specify how the missiles have been used, arguing the details would hinder operational effectiveness. However, defence sources say the drones are used against a wide variety of targets, especially "high-value targets" – a reference to Taliban commanders. They are often called in by British special forces and army commanders on the ground.

Missiles from a US drone flying over South Waziristan in Pakistan's tribal region are believed to have killed Baitullah Mehsud, the Pakistani Taliban chief last year. Highly publicised CIA attacks on Taliban and al-Qaida suspects on both sides of the Pakistan-Aghanistan border have been criticised for killing civilian bystanders and violating Pakistan's sovereignty.

The MoD says there have been no reports of RAF drones killing Freedocivilians.

The rules governing the firing of the Reapers' missiles "are no different to those used for manned combat aircraft, the weapons are all precision guided and every effort is made to ensure the risk of collateral damage and civilian casualties is minimised", a defence official said.

There has been a long debate within defence circles about the legality of firing weapons from such a distance and about the authority given to drone controllers.

Chris Cole, director of the interfaith peace campaign Fellowship of Reconciliation, who used freedom of information legislation to shed light on the Reapers, said: "Drones are the latest in a long line of new weapons used in the mistaken belief that they will provide a clean and tidy solution to a conflict – time and again history has proved that this is a myth."

He added: "We have a number of serious concerns not least because there is a picture beginning to emerge of high civilian casualties. In addition, the use of armed drones to target specific individuals could amount to summary or arbitrary execution."

Philip Alston, a UN human rights special rapporteur, warned in October that the US use of drones to kill militants in Afghanistan and Pakistan may violate international law. He called on the US to explain the legal basis for killing individuals with its drones. Many US intelligence officials view the Predator drones as their most important weapon against al-Qaida.CIA director Leon Panetta called them the "only game in town" last year.

The RAF is also flying small manned twin turboprop Beechcraft King Air planes to complement surveillance missions undertaken by the unmanned Reapers. They are equipped with sophisticated infrared radar and antennae for electronic and communications eavesdropping.

The MoD bought an initial batch of six Reaper drones from the US firm General Atomics, at a reported cost of £6m. One of the drones crashed in Afghanistan in 2008 in what defence secretary Bob Ainsworth called "a forced landing