The US government's refusal to offer a legal rationale for using unmanned drones to kill suspected militants in Pakistan could result in CIA officers facing prosecution for war crimes in foreign courts, a legal expert has told lawmakers.

"Prominent voices in the international legal community" were increasingly impatient with Washington's silence on the CIA's bombing raids in Pakistan and elsewhere, Kenneth Anderson, a law professor at American University, told a congressional panel on Tuesday.

Lawyers at the US State Department and other government agencies were concerned that the administration has "not settled on what the rationales are" for the drone strikes, he said.

"And I believe that at some point that ill serves an administration which is embracing this," said Anderson.

The law professor said he believes the drone strikes are legal under international law, based on a country's right to self-defense, and urged the US administration to argue its case publicly.

President Barack Obama has spoken about taking the fight to the enemy and denying safe havens to extremists, and US officials privately tout the drone raids against Al-Qaeda and Taliban leaders along the Pakistan border as highly effective.

But the administration declines to discuss the raids openly and has yet to publicly declare the legal justification for hunting down terror suspects in Pakistan and around the world.

"Now, maybe the answer is: This is all really terrible and illegal and anybody that does it should go off to the Hague. But if that's the case, then we should not be having the president saying that this is the greatest thing since whatever. That seems like a bad idea," Anderson said.

The congressional hearing broached a sensitive subject that is usually discussed by lawmakers and officials in closed sessions out of public view.

Human rights activists and some legal experts charge the drone strikes in Pakistan, outside of a traditional battlefield, amount to extrajudicial executions.

In written testimony, Anderson told the subcommittee of the House Oversight and Government Reform committee that officials and legal advisers at the CIA or the national security council who create "target lists" could face possible charges abroad over the drone war.

"It is they who would most likely be investigated, indicted, or prosecuted in a foreign court, as, the US should take careful note, has already happened to Israeli officials in connection with operations against Hamas," he wrote.

"The reticence of the US government on this matter is frankly hard to justify, at this point," he added.

The drone strikes have surged since Obama took office more than a year ago and have become the weapon of choice in Washington's fight against Al-Qaeda, despite concerns over civilian casualties and public anger in Pakistan.

Representative John Tierney, chair of the subcommittee on national security, said that the drone war raised an array of unanswered questions, including "if the United States uses unmanned weapons systems, does that require an official declaration of war or an authorization for the use of force?"

The American Civil Liberties Union last week filed a Freedom of Information Act lawsuit demanding the State Department and other agencies disclose the legal basis for carrying out assassinations overseas with unmanned aircraft.

The lawsuit asks for information on when, where and against whom drone strikes can be authorized, according to the ACLU.

Unmanned aircraft are also used for strikes in the wars in Iraq and Afghanistan, but those raids -- carried out on a recognized battlefield by the military and not by the CIA -- do not raise the same legal dilemmas, experts said.

The military has elaborate procedures to ensure bombing raids are carried out in line with international law and carefully vetted by commanders, with military lawyers providing advice, said Peter Singer, author of "Wired for War."

"The challenge is the use on the intelligence side," said Singer.

It was unclear if CIA civilian officers had the same training and experience to conduct an air war and adhere to international law, he said, adding that little was known about the spy agency's drone campaign.