Secretary of State Hillary Clinton promised Wednesday that the United States would use the Internet and social media in its fight against Al-Qaeda and other extremist Islamist groups.

The outgoing top diplomat said the US State Department had "started two organizations to deal with countering violent extremism" on the Internet, and would use tools such as Twitter to combat online anti-Americanism.

"Social media is a great tool," Clinton told lawmakers when speaking to Congress about the deadly attack on a US mission in Benghazi, Libya four months ago.

The new operation she has set up is "staffed with inter-agency experts," and "I'm not saying anything that's classified, but it's beginning to try to respond to Al-Qaeda and other jihadist propaganda.

"If they put up a video which talks about how terrible Americans are, we put up a video which talks about, you know, how terrible they are," Clinton said.

"We're trying to meet them in the media channels (in which) they are communicating with people."

Referring back to the Cold War era during "our fight against international communism," Clinton said media tactics had worked well and that in 2013 the United States would confront a similar challenge with an up-to-date strategy.

"We did a lot of things really well. I mean, we kept people's hopes alive, we communicated with freedom lovers and advocates behind the Iron Curtain. We did it through media, we did it through our values."

Clinton's testimony came on the eve of Thursday's Senate hearing to confirm her successor, John Kerry, who is expected to be easily voted in and could take over within days.

The outgoing Secretary insisted there was no cover-up of events in Benghazi, when dozens of heavily-armed Al-Qaeda-linked militants overran the US mission compound and a nearby CIA-run annex.

The incident set off an eight-hour firefight in which four people, including US ambassador Chris Stevens, were killed.