The CIA is secretly using an airbase in Saudi Arabia to conduct its controversial drone assassination campaign in neighbouring Yemen, according to reports in the US media.
Neither the Saudi government nor the country's media have responded to reports revealing that the drones that killed US-born cleric Anwar al-Awlaki and his son in September 2011 and Said al-Shehri, a senior al-Qaeda commander who died from his injuries last month, were launched from the unnamed base.
Iranian state media highlighted the story, which is also likely to be seized upon by jihadi groups. Saudi Arabia has previously publicly denied co-operating with the US to target al-Qaeda in Yemen.
Evidence of Saudi involvement risks complicating its relationship with the government in Sanaa and with Yemeni tribal leaders who control large parts of the country.
Disclosure of the Saudi co-operation comes ahead of the appearance of architect of the drone programme John Brennan's appearance before the US Senate confirmation hearing to become CIA director.
The drone issue is sensitive in Saudi Arabia because of the unpopularity of US military bases, which were thought to have been largely removed after the 2003 invasion of Iraq.
Saudi Arabia is home to the Muslim holy cities of Mecca and Medina and the continued presence of US troops after the 1991 Gulf War was one of the stated motivations behind al-Qaeda's 9/11 terrorist attacks and the Khobar Towers bombing five years earlier.
The revelation is an embarrassment to the White House, which pressurised the Washington Post and some other news organisations to suppress the information for 12 months on national security grounds. The timing is also unfortunate for Mr Obama because the killings of Awlaki and his son have contributed to demands in Congress for greater transparency by the White House over the legal basis for drone attacks on US citizens.
Senators who are expected to quiz Mr Brennan at his confirmation hearing today about the drones policy are pressing the White House to release the detailed legal opinion justifying the targeting of Americans, as well as the broader policy that permits Mr Obama and some other officials to sign off on a "kill list" of named targets.
On Tuesday, NBC made public an administration document dating from 2011 justifying the killing of US citizens who hold senior positions in al-Qaeda and who pose an "imminent threat of violent attack" against America. But some members of Congress said the document left many questions unanswered, and are requesting to see a more detailed 50-page memorandum from the White House office of legal counsel.
Eleven Senators have written to the White House hinting at a prolonged fight over the appointment of Mr Brennan if the administration does not co-operate with its request.
Some senators have also raised questions about civilian casualties and whether the drone attacks are a recruiting tool for the US's enemies.
The confirmation hearing is also expected to throw some light on the future use of drones, which Mr Obama has rapidly expanded in Pakistan, Afghanistan, Yemen and Somalia. Mr Brennan had helped forge the policy from his White House basement office, and persuaded the president of its value. Senators are likely to want to know where Mr Brennan will take the strategy as head of the CIA, which carries out drone attacks in parallel with the US military.
The CIA's use of drones has come under challenge as a breach of international law because the agency is not a recognised military force.