US Secretary of State John Kerry pledged Wednesday that nobody had lied about last year's attack on a US mission in Libya that killed four Americans, and he urged lawmakers to move on.

The former senator found the tables turned as he was grilled about the hot-button issue by his old congressional colleagues during his defense of the State Department's 2014 budget request at the House foreign affairs committee.

"I don't think anybody lied to anybody," Kerry insisted about the attack in Benghazi, in eastern Libya, which sparked a ferocious backlash from Republicans during the 2012 US presidential race.

Kerry also said he would not investigate statements made by Susan Rice, the US Ambassador to the United Nations in the days after the September 11 attack last year.

But amid persistent questioning from congressmen fuming that they still had not been given all the information they sought, Kerry vowed to appoint someone within the next 24 hours to work with them.

"Let's figure out what it is that's missing, if it's legitimate or isn't," he said. "Let's find out exactly, together, what happened, because... we got a lot more important things to move on to and get done."

Republicans have alleged that President Barack Obama's administration sought to cover up details of the Benghazi attack. Ambassador Chris Stevens was among the four Americans killed when Al-Qaeda-linked militants overran the mission.

A State Department probe initiated by Kerry's predecessor, Hillary Clinton, found security was "grossly inadequate" at the mission and a nearby CIA annex.

Answering questions as to why seven months on none of the perpetrators had been caught, Kerry said progress was being made.

"We have identified people. And they are building a case. You know, we're going through the tedious, laborious and very difficult process of gaining evidence from a part of the country which is dangerous," he said.

Kerry also defended Rice, who had initially been seen as the favorite to succeed Clinton, until she appeared on Sunday political talk shows and said the Benghazi attack had been "spontaneous."

Asked whether he would support an investigation into Rice's comments, Kerry replied firmly: "No, because I don't believe it's necessary. Ambassador Rice has apologized for her mistaken comments, which were based on talking points that she was given.

"I am absolutely confident that Ambassador Rice did not purposefully mislead anybody. She was using the talking points and there was confusion in the early hours."

But Kerry reiterated he would work with lawmakers to answer their questions. "I do not want to spend the next year coming up here talking about Benghazi," he added.