US Air Force via Reuters A Predator drone is shown in an undated photo from the Air Force.

Michael Reynolds / EPA U.S. Attorney General Eric Holder addresses the National Association of Attorneys General in Washington, D.C., on Feb. 26.

The Obama administration has "no intention" of carrying out drone strikes against suspected terrorists in the United States, but could use them in response to “an extraordinary circumstance” such as the 9/11 terror attacks, according to a letter from Attorney General Eric Holder obtained by NBC News.

Sen. Rand Paul, R-Ky., who received the March 4 letter from Holder, called the attorney general’s refusal to rule out drone strikes in the U.S. “more than frightening.”

The letter from Holder surfaced just as the Senate Intelligence Committee was voting 12-3 to approve White House counterterrorism adviser John Brennan to be CIA director. The vote came after the White House agreed to share additional classified memos on targeted drone strikes against U.S. citizens overseas.

Paul had threatened to hold up Brennan's confirmation on the floor of the Senate if the administration did not clarify whether targeted drone strikes could be used inside the U.S.

In his letter, Holder called the question of drone strikes inside the U.S. "entirely hypothetical, unlikely to occur and we hope no president will ever have to confront. … As a policy matter, moreover, we reject the use of military force where well-established law enforcement authorities in this country provide the best means for incapacitating a terrorist threat."

But Holder then appeared to leave the door open to such strikes in extreme circumstances.

Read the full letter

"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 authorize the military to use lethal force within the territory of the United States. For example, the president could conceivably have no choice but to authorize the military to use such force if necessary to protect the homeland in the circumstances of a catastrophic attack like the ones suffered on Dec. 7, 1941 and Sept. 11, 2001."

In a statement, Paul said, “The U.S. attorney general’s refusal to rule out the possibility of drone strikes on American citizens and on American soil is more than frightening – it is an affront the Constitutional due process rights of all Americans.”

Paul told NBC News that the response by Holder could lead to a situation where “an Arab-American in Dearborn (Mich.) is walking down the street emailing with a friend in the Mideast and all of a sudden we drop a drone” on him. He said it was “really shocking” that President Barack Obama, a former constitutional law professor, would leave the door open to such a possibility.

Paul said he will filibuster Brennan’s confirmation over the issue but acknowledged “we probably can’t stop him.” He did say, however, he intends to co-sponsor a bill with Republican Sen. Ted Cruz of Texas, to be introduced in the next few days, that would bar the president from using drone strikes in the U.S.

Related stories:

Justice Department memo reveals legal case for drone strikes on Americans

Senate panel votes to move Brennan's CIA nomination forward

More from Open Channel:

Follow Open Channel from NBCNews.com on Twitter and Facebook