Ten months to the day after four Americans, including an ambassador, died in an attack on a US mission in Libya, Republican lawmakers voiced anger that no-one was charged.

And they sought once again to back up accusations that the Obama administration deliberately tried to cover-up what happened when dozens of heavily-armed militants overran the mission in Benghazi and a nearby CIA annex.

Former State Department spokeswoman Victoria Nuland was in the spotlight during her nomination hearing to be the next assistant secretary of state for European and Eurasian affairs.

She voiced "frustration" that so far no charges have been brought into the killings of the four men, including ambassador Chris Stevens, although Al-Qaeda linked militants are thought to have carried out the attack.

The "American people deserve the truth, this body deserves the truth, those of us who were friends of the victims, as I was, deserve the truth," she said.

But Nuland said she had no inside knowledge of the investigation.

Both President Barack Obama and then secretary of state Hillary Clinton have vowed to hunt down the perpetrators.

"Today, July 11th, it has now been exactly 10 months since the attacks. To your knowledge, are we any closer to identifying and bringing those terrorists to justice?" asked Senator John Barrasso.

"I share your frustration," replied Nuland. "As I said, as a citizen, I want to know what happened, as well."

But she added: "I am not privy to information about how the investigation is going."

Senator Marco Rubio offered a half-apology for her grilling over the Benghazi attack, saying "there's very little debate on this committee about your qualifications to serve in this post."

The only reason she was getting asked about Benghazi was because with no further hearings planned "you're, quite frankly, the only witness we have."

Although Nuland was not in a policy position, in 100 pages of emails released by the White House in May, she is seen to have sought guidance from the CIA about some of the talking points, later used by UN ambassador Susan Rice on political talkshows the Sunday that immediately followed the killings.

Nuland said in one email that including a reference to previous attacks on foreigners in Benghazi would be "abused" by members of Congress to claim the State Department ignored warnings of an impending attack.

She also wrote that edits made to the script "don't resolve all of my issues or those of my building leadership."

Explaining her role, Nuland told the Senate Foreign Relations committee that as the State Department spokesman her "responsibilities were to ensure consistency of our public messaging but not to make policy."

When she saw the proposed talking points she was concerned they "indicated a significant evolution beyond" what she had been allowed to say just a few hours earlier at her daily press briefing.

She said she also felt the talking points were inconsistent, possibly prejudicial to the FBI investigation and might also be inaccurate.

Nuland also confirmed that then secretary of state Clinton was at the State Department late into the night of the September 11 attack as the events unfolded and that at one point they had worked together on a statement.