The US intelligence community said that a deadly assault on a US consulate in Libya was a planned attack linked to Al-Qaeda, but stressed that "many unanswered questions" remained.

"It remains unclear if any group or person exercised overall command and control of the attack, and if extremist group leaders directed their members to participate," Shawn Turner, spokesman for the Office of the Director of National Intelligence, said in a statement Friday.

"We do assess that some of those involved were linked to groups affiliated with, or sympathetic to Al-Qaeda."

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

US Ambassador to Libya Christopher Stevens and three other Americans were killed in the assault.

Both Defense Secretary Leon Panetta and top US diplomat Hillary Clinton have called the assault a "terrorist attack," with the Pentagon chief also suggesting that it took days for the US government to conclude extremists had launched an orchestrated assault.

"As we learned more about the attack, we revised our initial assessment to reflect new information indicating that it was a deliberate and organized terrorist attack carried out by extremists," Turner explained.

According to US media reports, the militants involved in the attacks belonged to a group called Ansar al Sharia. Its members were reportedly in contact with an Al-Qaida offshoot known as Al-Qaida in the Islamic Maghreb.

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 other Americans.

But Turner stressed that despite "progress" made in the investigation, "there remain many unanswered questions."

President Obama has dispatched a Federal Bureau of Investigation team to Benghazi to probe the killings of the ambassador and other Americans, but media reports say the team has not been able to reach the city because it remains too dangerous for Westerners.

As a result, the investigators remain stuck at the US Embassy in Tripoli, the reports said.

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

The US military's top officer, General Martin Dempsey, also said 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 said the attack arose out of a spontaneous protest against an amateur anti-Islam Internet video made in the United States.

The US ambassador to the United Nations, Susan Rice, on September 16 described it as a "spontaneous attack" that took place on the 11th anniversary of the September 11, 2001 terror strikes on the United States.

On Friday a top Republican congressman to call for Rice to resign, charging that she misled Americans over the assault by dismissing suggestions that it was a planned terror operation.

"I think Susan Rice should resign. She is America's foreign policy spokesman to the world as ambassador to the UN," Congressman Pete King of New York told National Review Online.

But earlier this week the White House dismissed Republican criticism as mere politics.

White House spokesman Jay Carney said the information given to the public about the attack was based on "the best intelligence we've had."