US Secretary of State Hillary Clinton warned of the challenges posed by rising militancy after the Arab Spring as she appeared before US lawmakers Wednesday to be grilled about a deadly attack.

"Benghazi didn't happen in a vacuum," Clinton said at the start of a Senate hearing into the September 11 assault on a US mission in eastern Libya.

"The Arab revolutions have scrambled power dynamics and shattered security forces across the region," she told the Senate Foreign Relations committee called to review the lessons learned from the Benghazi attack, in which US ambassador Chris Stevens and three other Americans were killed.

She choked up as she described welcoming the fallen diplomats home, when their bodies arrived in flag-draped coffins at the Andrews Air Force base.

"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 said, choking back a sob.

She warned lawmakers, however, that US diplomacy could not pull back in face of the new challenges posed by the evolving geopolitical landscape, saying the United States had to meet a "changing threat environment."

"We cannot afford to retreat now. When America is absent, especially from unstable environments, there are consequences. Extremism takes root, our interests suffer, and our security at home is threatened," she said.

Clinton also highlighted "instability in Mali," saying it "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."

Despite keeping a low profile after a long period of ill health in her final weeks in office, Clinton is keen to draw a line under the deadly September 11 Benghazi assault, which triggered a political storm in the United States.

The hearings have also taken on added urgency as Washington reels from last week's attack on a remote Algerian gas plant, in which three Americans were killed.

Clinton was initially set to testify in December after a scathing inquiry blamed "grossly inadequate" security at the outpost in Benghazi for failing to protect staff there.

But she was forced to send in her two deputy secretaries instead when she fell ill with a stomach bug. She later suffered a concussion in a fall, and a blood clot.

Her testimony to two congressional committees now comes on the eve of a Senate hearing to confirm her successor, Senator John Kerry, who is expected to be easily voted in and could take over within days as the top US diplomat.