U.S. Secretary of State Hillary Rodham Clinton delivered fiery rejoinders to Republican critics of the Obama administration's handling of the deadly attack on a U.S. mission in Benghazi, facing off with lawmakers who included potential 2016 presidential rivals.

At times emotional and frequently combative, Clinton rejected Republican suggestions in two congressional hearings that the administration tried to mislead the country about the Sept. 11 attack that killed Chris Stevens, the U.S. ambassador to Libya, and three other Americans. She insisted the State Department is moving swiftly and aggressively to strengthen security at diplomatic posts worldwide.

U.S. Secretary of State Hillary Rodham pounds her fist as she testifies on Capitol Hill in Washington on Wednesday before the Senate Foreign Relations Committee. (Pablo Martinez Monsivais/Associated Press)

In her last formal testimony before Congress as America's top diplomat — but perhaps not her last time on the political stage — Clinton once again took responsibility for the department's missteps and failures leading up to the assault. But she also said that requests for more security at the diplomatic mission in Benghazi didn't reach her desk, and reminded lawmakers that they have a responsibility to fund security-related budget requests.

Her testimony followed more than three months of Republican charges that the Obama administration ignored signs of a deteriorating security situation and cast an act of terrorism as mere protests over an anti-Muslim video in the heat of a presidential election. U.S. officials suspect that militants linked to al-Qaeda carried out the attack.

Clinton at times defiant, emotional

Three weeks after her release from a New York hospital — admitted for complications after a concussion — Clinton was at times defiant, complimentary and willing to chastise lawmakers during more than 5 1/2 hours of testimony before two separate committees.

She tangled with some who could be rivals in 2016 if she decides to seek the presidency again as she did in 2008, losing the Democratic nomination to President Barack Obama.

'I stood next to President Obama as the Marines carried those flag-draped caskets off the plane at Andrews. I put my arms around the mothers and fathers, sisters and brothers.'—Hillary Clinton, U.S. Secretary of State

Her voice cracking at one point, Clinton said the attack and the aftermath were highly personal tragedies for the families of the victims who died — Stevens, Sean Smith, Tyrone Woods and Glen Doherty — as well as herself.

"I stood next to President Obama as the Marines carried those flag-draped caskets off the plane at Andrews. I put my arms around the mothers and fathers, sisters and brothers, sons and daughters and the wives left alone to raise their children," she told the Senate Foreign Relations Committee at a packed hearing.

Clearly annoyed with Republican complaints about the initial explanation for the attack, she rose to the defence of UN Ambassador Susan Rice, who was vilified for widely debunked claims five days after the attack that protests precipitated the raid rather than terrorism.

Clinton said, "People were trying in real time to get to the best information." And she said her own focus was on looking ahead on how to improve security rather than revisiting the talking points and Rice's comments.

As Republican criticism of her mounted, Rice was forced to withdraw her name as a possible successor to Clinton.

Republicans challenge Clinton

Sen. Ron Johnson, a Republican, pressed her on why "we were misled that there were supposedly protests and something sprang out of that, an assault sprang out of that."

"With all due respect, the fact is we had four dead Americans," she said, her voice rising and quivering with anger as she and Johnson spoke over each other.

'Had I been president and found you did not read the cables from Benghazi and from Ambassador Stevens, I would have relieved you of your post.'—Sen. Rand Paul

"Was it because of a protest? Or was it because of guys out for a walk one night decided they would go kill some Americans? What difference, at this point, does it make? It is our job to figure out what happened and do everything we can to prevent it from ever happening again, Senator."

If Johnson's comments drew an irritated response from Clinton, she notably ignored Sen. Rand Paul, a Republican, when he said he would have fired her if he had been in charge and found that she had not read cables from her team in Libya asking for more security. Paul is a potential 2016 presidential candidate.

"Had I been president and found you did not read the cables from Benghazi and from Ambassador Stevens, I would have relieved you of your post," Paul said. "I think it's inexcusable."

A burnt car in front of the U.S. consulate in Benghazi, Libya, after an attack in September last year that killed four Americans. (Mohammad Hannon/Associated Press)

Clinton and other officials have testified that requests for additional security did not reach her level, and a scathing independent review of the matter sharply criticized four senior State Department officials who have been relieved of their duties.

"I did not see these requests. They did not come to me. I did not approve them. I did not deny them," she said.

Later, before the House Foreign Affairs Committee, Republican Rep. Jeff Duncan of South Carolina repeatedly challenged Clinton's claim to have looked at the tragedy with "clear eyes," saying she should have personally ensured security at the mission.

He said Clinton had "let the consulate become a death trap" in denying requests for additional security and called it "malpractice."

Clinton said she could have let the review board's report remain classified and told Congress "goodbye" before leaving office. But she said, it's "not who I am. It's not what I do."

Clinton resigning from post this week

Absent from the Senate hearing was Sen. John Kerry, a Democrat and the man tapped to succeed Clinton, who is leaving the administration after four years. Kerry, defeated by George W. Bush in the 2004 presidential election, is expected to win swift Senate approval. Clinton is to introduce him at his confirmation hearing on Thursday.

U.S. Secretary of State Hillary Rodham testified in a packed hearing room before lawmakers on Wednesday over the deadly attack on the U.S. diplomatic mission in Benghazi, Libya last year. (Manuel Balce Ceneta/Associated Press)

A former New York senator and the wife of former President Bill Clinton, she is a polarizing figure but is ending her tenure at the State Department with high favourability ratings. A poll last month by the Pew Research Center for the People & the Press found 65 per cent of Americans held a favourable impression of her, compared with 29 per cent unfavourable.

On the panel at the Senate hearing were two possible 2016 Republican presidential candidates — Florida's Marco Rubio and Paul, a new member of the committee — as well as John McCain, who was defeated by Obama in November 2008.

At the House hearing, Republican members pressed her on why cables and other memos about security deficiencies in Benghazi seemed to be ignored.

"The dots here were connected ahead of time. The State Department saw this was coming," said Rep. Ed Royce, a Republican and the chairman of the panel. "The State Department didn't act."

Clinton told senators the department is implementing the 29 recommendations of the review board and going beyond the proposals, with a special focus on high-threat posts.

"Nobody is more committed to getting this right," she said. "I am determined to leave the State Department and our country safer, stronger, and more secure."

Clinton, 65, had been due to testify in December but postponed her appearances after fainting, falling and suffering a concussion while recovering from a stomach virus that left her severely dehydrated. She was then diagnosed with a blood clot near her brain and returned to work only on Jan. 7.