US Defense Secretary Leon Panetta said Thursday a deadly assault on a US consulate in Libya was a planned "terrorist" attack but that it remained unclear if Al-Qaeda had a hand in the incident.

President Barack Obama's administration has offered varied explanations as to who may have been behind the September 11 attack on the American diplomatic mission in Benghazi, drawing criticism from Republican opponents weeks before a US presidential election.

Panetta, repeating Secretary of State Hillary Clinton's comments on Wednesday, said the assault was a "terrorist attack" and also suggested that it took days for the US government to conclude extremists had launched an orchestrated assault.

"As we determined the details of what took place there, and how that attack took place, that it became clear that there were terrorists who had planned that attack, and that's when I came to that conclusion," he said.

"It took a while to really get some of the feedback from what exactly happened at that location."

Some Republican lawmakers have alleged that the Obama administration knew almost immediately afterward that Al-Qaeda was involved in the Benghazi attack, which killed the US ambassador to Libya and three of his staff.

But Panetta said it was too soon to say whether Al-Qaeda or Al-Qaeda-linked groups had a role in the incident.

"As to who was involved, what specific groups were involved, I think the investigation that is ongoing hopefully will determine that," said the former CIA director.

The US military's top officer, General Martin Dempsey, told the same press conference that there had been no warning of a direct threat to the American mission in Benghazi before the attack.

"There was a thread of intelligence reporting that groups in the environment in... eastern Libya were seeking to coalesce, but there wasn't anything specific and certainly not a specific threat to the consulate that I'm aware of," the general said.

The State Department initially maintained the attack arose out of a spontaneous protest against an anti-Islam Internet video made in the United States.