President Barack Obama could in "extraordinary circumstances" use deadly drone strikes against Americans inside the United States, the country's attorney general has said.

The use of unmanned drones by the US has escalated sharply under the Obama administration, with the Democratic President using deadly strikes to go after militants overseas. Scores of civilian casualties have been reported as a result, and the program has generated anger among officials.

John Brennan has overseen the expansion. His nomination triggered a wave of questions, including one from Republican Senator Ran Paul who asked the administration if the President had the "power to authorise lethal force, such as a drone strike, against a US citizen on US soil, and without trial."

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The response: under "extraordinary circumstances," according to attorney general Eric Holder, who said the administration had not carried out drone strikes in the US and had no intention of doing so. "The question you have posed is therefore entirely hypothetical, unlikely to occur and one we hope no President will ever have to confront," he wrote to Senator Paul.