During the George W Bush administration, the US conducted around 50 drone strikes to kill suspected terrorists. The Obama administration, however, has ordered around 500 strikes, according to the Bureau of Investigative Journalism, which tracks the use of drones by the US military and CIA.

The bulk of the drone strikes have taken place in Pakistan, where they have become a major factor in spurring anti-American sentiment, but they have also been conducted in Yemen, Somalia and Afghanistan.

The numbers of US drone strikes appear to have decreased significantly over the past two years, especially in Pakistan. This is partly the result of political blowback, but also reflects the difficulty of obtaining reliable intelligence in Yemen after the collapse of its government.