US military leaders ruled out sending in forces during the attack on an American consulate in Libya last month because of a lack of reliable intelligence, Defense Secretary Leon Panetta said Thursday.

Although forces were on alert and ready to launch an operation if needed, the US military commander for Africa, General Carter Ham, the chairman of the US Joint Chiefs of Staff, General Martin Dempsey, and Panetta all decided against any intervention as they had no clear picture of events unfolding in Benghazi, he said.

"There's a basic principle here, and the basic principle is that you don't deploy forces into harm's way without knowing what's going on, without having some real-time information about what's taking place," Panetta told a news conference.

"And as a result of not having that kind of information, the commander who's ...in that area, General Ham, General Dempsey and I felt very strongly that we could not put forces at risk in that situation."

He acknowledged some were second-guessing how the administration had responded but added: "This happened within a few hours and it was really over before, you know, we had the opportunity to really know what was happening."

The attack on September 11 by dozens of heavily-armed militants kept security guards at bay for hours and left four Americans dead, including US ambassador Chris Stevens.

The incident has become the subject of bitter dispute in the US presidential campaign, with Republican hopeful Mitt Romney and lawmakers in Congress accusing the White House of botching the response to the assault and trying to play down the role of Islamist extremists.

President Barack Obama's administration initially blamed the assault on protesters before concluding it was a planned attack by extremists.

Panetta noted that the Pentagon sent platoons of Marines to Tripoli and to Yemen to bolster security at US missions after the attack, and deployed naval ships off the coast of Libya.

Dempsey, the country's highest-ranking military officer, told the same press conference that reviews by the Pentagon and State Department would examine how the assault was handled but he said the military had responded promptly.

"I can tell you sitting here today that I feel confident that our forces were alert and responsive to what was a very fluid situation," Dempsey said.