David Petraeus pledged to confront the many threats facing America as he took the reins of the CIA, a decade after the 9/11 attacks that led to two wars where the former general made his name.

Petraeus was sworn in to lead the civilian covert war against Al-Qaeda having hung up his army uniform less than a week ago after a 37-year career that ended with years of difficult command roles in Iraq and Afghanistan.

Wearing a business suit and burgundy tie, Petraeus took the oath of office on a Bible at a ceremony led by Vice President Joe Biden in the Roosevelt room of the White House, instead of CIA headquarters in Langley, Virginia.

A voracious consumer of intelligence documents, Petraeus, 58, brings at a pivotal time to the agency his experience as a commander who melded military and intelligence operations to an extraordinary degree in Iraq and Afghanistan.

He succeeds Leon Panetta, now the defense secretary, ahead of a complete US troop pullout in Iraq later this year and a phased withdrawal from Afghanistan, both of which remain plagued by violence, in 2014.

Petraeus, pointing to his experience as a four star general but stressing that he knew he faced a different challenge in his new post as the nation's spy chief, said he would seek to make the CIA "ever better."

"I've found through hard experience that it's important to continually learn and adapt, to constantly assess situations, to identify and share lessons learned," he said in a statement released by the agency after the swearing in.

He later sent a note to all employees, praising "the talented men and women of the CIA, whose courage, ingenuity, and hard work have made a decisive difference in protecting the nation."

"I eagerly look forward to starting our work together, and to bringing our agency's unique strengths to bear on the many foreign threats and opportunities our country faces," he added.

Ambitious and supremely disciplined, Petraeus has sometimes irritated fellow commanders, earning the nickname "King David." But his successes on the battlefield won him fame and the confidence of US presidents.

Then president George W. Bush put him in command of US forces in Iraq in 2007 as the country was descending into civil war and Petraeus won acclaim in some quarters for salvaging the situation with a controversial troop surge.

Whether the decision to increase troop numbers turned the tide in Iraq is still under debate, with critics arguing that the real reason violence receded was because Al-Qaeda's brutal tactics alienated Sunni tribal leaders.

President Barack Obama turned to Petraeus to take command in Afghanistan in mid-2010 as security conditions deteriorated despite a surge of 30,000 extra US soldiers.

Before stepping down in July after nearly a year as head of the US-led force in Afghanistan, Petraeus claimed progress as American troops rolled back the insurgents in the south while Afghan security forces expanded.

But the Taliban remains resilient and US troop deaths in Afghanistan are at record levels just days from the 10-year anniversary of September 11, which prompted Bush to order an invasion to oust a government that was harboring Al-Qaeda chief Osama bin Laden.

The CIA has in recent years conducted a lethal and often controversial campaign of drone attacks in neighboring Pakistan that former chief Panetta contends has brought Al-Qaeda to the brink of strategic defeat.

But Petraeus also inherits tense relations with Islamabad and its intelligence services, which were infuriated by a clandestine May 2 raid by US Navy SEALs that killed bin Laden in a Pakistani garrison town.

Pakistan's Inter-Services Intelligence Agency reacted by arresting local informants who helped the CIA prepare for the raid, and American suspicions of Pakistani complicity in hiding bin Laden have run high.

As CIA director, however, Petraeus's responsibilities extend far beyond Al-Qaeda to some of the hardest and most complex targets facing US intelligence -- North Korea, Iran and China among them.

When Petraeus retired from the military on August 30, Admiral Michael Mullen, chairman of the Joint Chiefs of Staff, hailed him as one of the "giants" in US military history.