Attorney General Eric Holder made clear on Thursday that a US president does not have the power to order a drone strike against a "non combatant" American inside the United States.

Holder's clarifying comment came in a short letter to Senator Rand Paul, who mounted a 13-hour, non-stop filibuster in the Senate demanding answers from the administration on the scope of drone policy.

"Does the President have the authority to use a weaponized drone to kill an American not engaged in combat on American soil?" Holder asked in a letter to Paul. "The answer to that question is 'no.'".

Paul, an idiosyncratic, libertarian Republican, mounted the filibuster delaying tactic into the early hours of Thursday against the nomination John Brennan to head the CIA.

He had demanded a clarification from the administration to ensure "they are not going to kill non-combatants in America" bringing a new US perspective to a row raging over the use of US drones against terror suspects abroad.

On Tuesday, Holder had said that, while Obama had "no intention" of ordering drone strikes on US soil, the scenario could be possible if there was an "extraordinary circumstance" such as an attack similar to 9/11.

Paul said after he received the letter that it showed his battle was worthwhile.

"The reason this is important is often drones are used overseas (against) people who are not actively engaged in combat," he told reporters.

"I'm not saying they're not bad people or they might have previously been in combat, but the thing is, we have to have a higher standard in our country."

Paul's action won praise from some Republicans and those conservatives who see him as the heir to his father Ron Paul's quirky coalition in presidential campaigns.

But critics questioned whether he was truly engaging on a point of principle and worried that he was scaremongering, since no US official had seriously mooted the use of drones against US citizens on home soil.

"The president has not and would not use drone strikes against American citizens on American soil," White House spokesman Jay Carney said.

"Those legal authorities that exist to use lethal force are bound by and constrained by the law and the Constitution.

"The issue here isn't the technology. The method does not change the law. The president's sworn an oath to uphold the Constitution, and he is bound by the law.

"Whether the lethal force in question is a drone strike or a gunshot, the law and the Constitution apply in the same way."