Top US and Libyan officials had offered divergent assessments of the attack that killed the US ambassador and three other Americans last week, with Libyan officials it was pre-planned by foreign extremists and local sympathizers.

But the US ambassador to the United Nations, Susan Rice, said Sunday it began with a spontaneous protest over an anti-Islamic video that had also set off similar protests in Egypt, leading to the storming of the US embassy there.

"We have provided information about what we believe was the precipitating cause of the protests and the violence based on the information that we have had available," White House spokesman Jay Carney said.

"There is an ongoing investigation. The FBI is investigating, and that investigation will follow the facts wherever they lead."

Rice said on Fox News Sunday at the weekend that a demonstration over the film that sparked unrest across the Arab world, outside the consulate in Benghazi was joined by those with "extremist ties" with heavy weapons.

"But we don't see at this point signs this was a coordinated plan, premeditated attack. Obviously, we will wait for the results of the investigation and we don't want to jump to conclusions before then."

Carney noted Tuesday that there were a number of "violent groups" in Libya as the authorities struggle to assert control in a chaotic revolutionary period as well as vast numbers of weapons.

He referred questions about whether security was adequate at the Benghazi consulate ahead of the demonstrations which coincided with the anniversary of the September 11 attacks in 2001 to the State Department.