The White House has bowed to demands from Congress to release its legal reasoning for the killing of US citizens by armed drones in an attempt to ease pressure on John Brennan, the architect of the drones strategy, at his Senate confirmation hearing as CIA director on Thursday.

The Associated Press reported that Barack Obama ordered that the 50-page legal rationale finally be given to the Senate intelligence committee as members' anger at the administration's refusal to share the classified document led to an implied threat from some senators to hold up Brennan's nomination.

The White House move is likely to defuse some of the tension at Brennan's hearing and possibly limit the scope of senators' questions on the sensitive issue of the president ordering the deaths of Americans in al-Qaida on the grounds they will have been provided with classified information.

The release of the legal opinion to the intelligence committee is the latest twist in an unusually dramatic runup to a nomination for a CIA director, which is normally a routine affair.

On Wednesday, US newspapers revealed that there is a secret CIA drone base in Saudi Arabia used to conduct a controversial assassination campaign in neighbouring Yemen. Washington and Riyadh were silent over the reports, which revealed that the drones that killed the US-born cleric Anwar al-Awlaki and his son in September 2011, and Said al-Shehri, a senior al-Qaida commander who died from his injuries last month, were launched from the unnamed base.

The revelation is an embarrassment to the White House, which pressured 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 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 Brennan at his confirmation hearing on Thursday about the drones policy have been at the forefront of pressing the White House to release the detailed legal opinion justifying the targeting of Americans, as well as the broader policy that permits 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-Qaida and who pose an "imminent threat of violent attack" against America. But some members of Congress said the document left many questions unanswered and pressed to see a more detailed 50-page memorandum from the White House office of legal counsel.

Earlier this week, 11 senators wrote to the White House hinting at a prolonged fight over the appointment of Brennan if the administration does not co-operate with its request.

"The executive branch's co-operation on this matter will help avoid an unnecessary confrontation that could affect the Senate's consideration of nominees for national security positions," the letter said.

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 Obama has rapidly expanded in Pakistan, Afghanistan, Yemen and Somalia. 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 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.

The revelation of the CIA drone base in Saudi Arabia was seized on by Iran, where state media highlighted the story. It is also likely to be grasped as an opportunity by jihadi groups. Saudi Arabia has previously publicly denied co-operating with the US to target al-Qaida in Yemen.

Evidence of Saudi involvement risks complicating its relationship with the Yemeni government in Sana'a, and with tribal leaders who control large parts of Yemen where al-Qaida operates.

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 Islamic 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-Qaida's 9/11 terrorist attacks and the Khobar Towers bombing five years earlier.

The date of the 1998 US embassy bombings in Kenya and Tanzania was eight years to the day after US troops were first sent to Saudi Arabia.

Osama bin Laden interpreted the prophet Muhammad as banning the "permanent presence of infidels in Arabia".

The last significant US military presence was at the Prince Sultan airbase in Khobar in the eastern province. The forces there were relocated to Qatar in September 2003 after 13 years, and at the height of the Iraq war it housed 10,000 US troops and 200 planes. It was supplanted as the Middle East's main American military air operations centre by Al Udeid Air Base in Qatar.

Still, the revelation is unlikely to have a significant impact inside the kingdom. Saudi Arabia has no independent media or expectations of government transparency, but there is no sympathy for the jihadis of al-Qaida targeted in Yemen. Saudi Arabia conducted its own successful campaign against al-Qaida, in effect destroying it by 2004. Its remnants moved to Yemen and formed al-Qaida in the Arabian Peninsula (Aqap), perhaps the most active of the group's regional "franchises".

Aqap has tried repeatedly to hit Saudi targets. In 2009, Mohammed Bin Nayef, the deputy interior minister and anti-terrorist chief, had an extraordinarily lucky escape when an al-Qaida suicide bomber from Yemen blew himself up in the minister's palace but left his target only lightly injured. Eight men accused of plotting attacks in Riyadh were arrested last September. Six of them were Yemenis.

"These planes are unmanned and unmarked, so there will not be the same impact as when American planes were flying from the Prince Sultan base," Mustafa Alani of the Gulf Research Centre in Dubai told the Guardian. "No one will say that the Americans are occupying the country.

"I don't think people care about this any more. Generally it is accepted in the region that the planes operated by the Americans are not doing a bad job in taking out al-Qaida leaders. There is no sympathy with al-Qaida, except a very small minority. Even in Yemen – apart from the collateral damage where civilians lose their lives– there is no objection to this type of operation.

"It has been rumoured for years that drones were taking off from the Arabian peninsula, so this is not shocking news except for the Iranians and jihadis. Otherwise it is not going register in public opinion."

US government requests to American media to refrain from disclosing the location of the CIA base were made, in part, because it could potentially damage counter-terrorism collaboration with Saudi Arabia.

Shehri, the deputy leader of Aqap, died last month of injuries sustained during a US drone strike in 2012.