US Secretary of State Hillary Rodham Clinton has spoken of her commitment to improving the security of American diplomats after the 11 September assault on the country's diplomatic mission in Benghazi, Libya that killed four Americans including ambassador Chris Stevens.

In her last appearance in Congress as secretary of state, Clinton said she took responsibility for the attack. "Nobody is more committed to getting this right. I am determined to leave the State Department and our country safer, stronger and more secure," she said.

Washington insiders were surprised at the emotional range displayed by the normally ice-cool Clinton.

Clinton's voice broke moments when she told the Senate foreign relations committee her commitment to the safety of diplomats is more than professional.

"It's personal," she said, describing the sight of the four returning coffins and the grieving families there to receive them.

Clinton said she directed the US response to the attack.

"I saw first-hand what Ambassador Thomas Pickering and former Chairman of the Joint Chiefs of Staff Admiral Mike Mullen called 'timely" and 'exceptional' coordination," Clinton said. "No delays in decision-making. No denials of support from Washington or from the military.

"I instructed our senior department officials and diplomatic security to consider every option - to break down the doors to the Libyan officials to get as much security support as we could," she added.

The departing secretary of state was supposed to testify in December, but her appearance was postponed by illness and then a concussion.

In her testimony, Clinton linked the events in Benghazi to broader regional factors, including the presence of al-Qaida affiliated groups in northern Mali.

"Benghazi didn't happen in a vacuum," she said. "The Arab revolutions have scrambled power dynamics and shattered security forces across the region. And instability in Mali has created an expanding safe haven for terrorists who look to extend their influence and plot further attacks of the kind we saw just last week in Algeria."

The wife of former president Bill Clinton said Mali is "a necessary struggle. We cannot let northern Mali [the area in which the terrorists are predominant] become a safe haven."

In the wake of the Benghazi attack, Clinton came under attack by Republicans who accused the Obama administration of ignoring signs of a deteriorating security situation in Libya, and downplaying an act of terror as a mere protest over an anti-Muslim video.

An Independent Accountability Review Board stated that "systematic failures" paved the way for the deadly attack. No protests preceded the assault.

Clinton also discussed the implementation of the Board's 29 recommendations. "I have accepted every one of the recommendations, asked they be implemented quickly, with clear timelines for completion," she said.

"There were inadequacies in the response. Those are the specific kind of recommendations we are currently implementing."