United States forces could launch a deadly drone strike against a target on US soil if there was an "extraordinary circumstance", Attorney General Eric Holder said in a letter to US Senator Rand Paul, a Republican.
In his letter to Paul, which Paul released, Holder stressed that US military and intelligence agencies currently have "no intention" of carrying out such an attack.
Paul branded Holder's refusal to comprehensively rule out such a drone strike "more than frightening".
Three Americans are known to have been killed in US drone strikes, including al-Qaeda propagandist Anwar al-Awlaki. They were targeted in Yemen, not on US soil, but questions have been raised about the legitimacy of the tactic.
Paul had sought information on President Barack Obama's authority to order lethal drone strikes as part of the confirmation process for John Brennan, Obama's pick to head the CIA.
"The question you have posed is … entirely hypothetical, unlikely to occur and one we hope no president will ever have to confront," Holder wrote, in a letter dated Monday.
"It is possible, I suppose, to imagine an extraordinary circumstance in which it would be necessary and appropriate under the Constitution and applicable laws of the United States for the president to authorise the military to use lethal force within the territory of the United States."
Holder cited the attacks of September 11, 2001, and on Pearl Harbour by the Japanese in 1941 as examples where use of such force might be justified.
Brennan, a 25-year CIA veteran, is known as the chief architect of the drone war, and he faced repeated questions at his confirmation hearing over the "targeted killings".
His nomination passed a key hurdle on Tuesday with the Senate Intelligence Committee voting by 12 votes to three to approve Brennan to head the CIA. A full Senate vote is expected this week.
The approval followed a White House decision to turn over to Congress several secret Justice Department memos that may have been used to justify Obama's ability to authorise the targeted killing of Americans.
Paul, who has long questioned the legality of the government's use of lethal force against US citizens, said Holder's response was "an affront [to] the Constitutional due process rights of all Americans".
Paul had also sought Brennan's views on drone strikes, and Brennan replied that the CIA had no authority to launch such attacks on US soil.