Top US and Libyan officials offered starkly different accounts Sunday about the attack on the US consulate in Benghazi that left the ambassador and three other Americans dead.

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

"People gathered outside the embassy (consulate) and then it grew very violent and those with extremist ties joined the fray and came with heavy weapons, which unfortunately are quite common in post-revolutionary Libya and that then spun out of control," Rice told "Fox News Sunday."

"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 (FBI) investigation and we don't want to jump to conclusions before then."

Announcing the arrest of 50 suspects, Libya's parliament chief, however, blamed the attack on a few foreign extremists who he said entered Libya from Mali and Algeria and pre-planned it with local "affiliates and sympathizers."

"The way these perpetrators acted, and moved... leaves us with no doubt that this was pre-planned, determined, predetermined," Mohammed al-Megaryef, president of the Libyan National Congress, told CBS News.

"It was planned, definitely, it was planned by foreigners, by people who entered the country a few months ago. And they were planning this criminal act since their arrival," he added.

Ambassador Chris Stevens is believed to have died from smoke inhalation after being trapped in the blazing diplomatic compound in Benghazi, which came under fire from rocket-propelled grenades, mortars and small arms for several hours.

Al-Qaeda in the Arabian Peninsula claimed the attack was revenge for the killing of the terror network's deputy leader Sheikh Abu Yahya al-Libi in a drone strike in June, but there is no clear evidence to support this claim.

US authorities initially leaned more toward the premeditated, well-planned assault angle, citing the fact that the attack came on the anniversary of 9/11.

But now they are more reticent, insisting that journalists wait for the results of the FBI investigation before leaping to any conclusions.

Rice's comments are the strongest indication yet that -- even if the United States does believe the assault was the work of a small band of extremists -- it has no evidence to suggest planning prior to the protests.

But leading Republican senator and top Obama administration critic, John McCain, said it was ridiculous to believe this was anything other than a planned assault by extremists.

"Most people don't bring rocket-propelled grenades and heavy weapons to a demonstration," McCain told CBS's "Face the Nation."

"That was an act of terror. And for anyone to disagree with that fundamental fact I think is really ignoring the facts."

The reluctance of the US officials to give details about the Libya probe betrays the determination of Washington to track down those who killed Stevens, the first US envoy killed in the line of duty since 1979.

US spies, Marines and drones are aiding the Libyan authorities in their effort, hampered by the chaotic blend of competing militia and tribal alliances in the wake of the revolution that ousted Moamer Kadhafi last year.

In Paris, French Foreign Minister Laurent Fabius told French TV that the American ambassador died under "absolutely atrocious conditions," suggesting new details could soon emerge.

Rice denied the suggestion that the United States seemed powerless to stop the anger spreading through the Muslim world at symbols of US influence, such as diplomatic missions, businesses and fast food restaurants.

She said the flare-up in the Middle East, North Africa, and some Asian countries was triggered solely by the amateurish movie lampooning the Prophet Mohammed and not due to waning US popularity in the Muslim world.

Rice called it a "direct result of a heinous and offensive video that was widely disseminated, that the US government had nothing to do with, which we have made clear is reprehensible and disgusting."