US Customs and Border Protection has grounded its drone fleet after a crew was forced to crash a pilotless craft off the coast of southern California, a spokesman said.
The crew operating the malfunctioning drone deliberately downed it in the Pacific Ocean last night after a mechanical problem.
The drone and systems on board were worth $12 million (£7.2m), the official said.
Defence Secretary Philip Hammond told ITV News' Deputy Political Editor Chris Ship that "you can never be 100% sure there won't be collateral damage", as the RAF unveiled its nerve centre of a controversial drone programme for the first time today.
Mr Hammond added: "We only know of one strike where there were civilian casualties. But of course, civilian casualties have also resulted from strikes by manned aircrafts. That is the nature of warfare".
RAF Wing Cdr Damian Killeen, the commanding officer of 13 Squadron has told ITV News that there is "absolutely no difference" between operating a drone and flying an aircraft in Afghanistan, as the RAF unveiled the nerve centre of its controversial drone programme for the first time.
For the first time US politicians heard from civilian victims of their controversial drone attack programme in north-west Pakistan.Read the full story ›
US accused of carrying out 'war crimes' in Yemen and Pakistan in major drone reportRead the full story ›
The US government only acknowledges its role in targeted killings in general terms, refusing to take responsibility for individual strikes or provide casualty figures, including civilian deaths, Human Rights Watch said in their report on drone attacks in Yemen
The Yemeni authorities have been almost as silent, the rights organisation said. Both governments have declined to comment on the six strikes that Human Rights Watch investigated.
An interactive map published by Amnesty International details recent drone attacks the human rights organisation says the US carried out in northwest Pakistan.
Amnesty International found evidence that a number of civilians, including an elderly woman and a group of young labourers, were killed in drone strikes in North Waziristan between January 2012 and August this year.
Researchers from Human Rights Watch have called on the US to assess the impact of drone attacks in Yemen, which they say are in "clear violation of the laws of war."
The calls come as Human Rights Watch and Amnesty International released two reports investigating who died as a result of the attacks in Yemen and Pakistan.
The US says it is taking all possible precautions during targeted killings, but it has unlawfully killed civilians and struck questionable military targets in Yemen.
Yemenis told us that these strikes make them fear the US as much as they fear al-Qaeda.
The US should investigate attacks that kill civilians and hold those responsible for violations to account. It's long past time for the US to assess the legality of its targeted killings, as well as the broader impact of these strikes on civilians.
Human Rights Watch are calling for the US government to assess the legality of drone strikes in Yemen, after their study of six such attacks found that the majority of those killed were civilians.
The strikes investigated by the rights group, between 2012 and 2013, found at least 57 of the 82 people killed were civilians; equivalent to 70% of the casualties.