WASHINGTON: US lawmakers have accused the White House of rebuffing their inquiries into CIA drone bombing raids abroad and vowed to assert more congressional oversight over the secretive drone war.

Both Republicans and Democrats on the House Judiciary Committee on Wednesday denounced President Barack Obama's administration for refusing to share key documents or details of the killings by armed, robotic aircraft.

"The need for oversight is clear," said Representative John Conyers, a Democrat and normally a staunch ally of the Obama administration.

Conyers and other members of the committee said it was unacceptable that the Attorney General, Eric Holder, or other officials from the Justice Department declined an invitation to appear at the hearing devoted to the drone campaign.

"I don't think the attorney general of the United States can decline to come before this committee on a subject that is so clearly within our jurisdiction," Conyers said.

A bipartisan group of lawmakers has asked the administration for a chance to review memos that outline the legal basis for killing Americans overseas suspected of having links to Al-Qaeda. But their requests have been mostly denied so far.

"The American people deserve to know and understand the legal basis under which the Obama administration believes it can kill US citizens, and under what circumstances," committee chairman Bob Goodlatte said.

Debate about the use of drones has been mounting following the September 2011 killing in Yemen of cleric Anwar al-Awlaki, a senior Al-Qaeda operative who was an American citizen.

At the hearing, lawmakers asked legal experts how the drone strikes might be better regulated by Congress or the courts, to provide a check on what they called potentially excessive presidential power.

Several committee members, including Goodlatte, said they were open to imposing more congressional or even judicial oversight over the drone campaign, possibly by reviewing the executive branch's decisions on drone attacks after, but not before, operations.

"For me, it's got to be retrospective, not prospective," Goodlatte said, as otherwise the president's ability to conduct a war against Al-Qaeda could be hampered.

He also said he favored Congress performing a review of the drone strikes instead of courts, as he worried that involving judges could interfere with a president's commander-in-chief role.

"I would prefer we review it here in Congress in some form rather than handing over war-fighting to the judicial branch."

Committee members and expert witnesses at the hearing for the most part did not question the legal basis for killing militants abroad who are deemed to be senior figures in the Al-Qaeda terror network, with which the United States is at war.

But they expressed concern about the intense secrecy around the strikes and questioned if sufficiently thorough procedures were in place when the administration decides what militant should be placed on a "kill list."

A former senior legal adviser during George W. Bush's presidency, John Bellinger, said Congress should consider adopting legislation offering a more detailed definition of the criteria needed to allow the targeting of terror suspects.

But he said the United States had to better explain to other governments the legal basis for its operations.

"No other country in the world has come out publicly and said that they actually agree with our position," Bellinger said.

And he acknowledged that the United States should brace itself for the day when another country, such as Russia or China, carries out its own drone killing outside of its borders, under a similar veil of secrecy.

Obama has promised to work with Congress to ensure the drone operations are "more transparent" and that the public understands the constraints and legal rationales underpinning the drone war.