Secretary of State Hillary Clinton will make a long-awaited appearance before lawmakers on January 22 to be quizzed about the deadly attack on a US mission in Libya, a senator said Tuesday.

Clinton had initially been due to testify to US lawmakers in late December after a scathing inquiry blamed "grossly inadequate" security at the diplomatic outpost in Benghazi for failing to protect staff there.

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

Ambassador Chris Stevens and three other Americans were killed when hordes of heavily-armed militants overran the compound and a nearby annex on September 11, unleashing a bloody and terrifying eight-hour assault.