The White House for the first time described the assault on the US consulate in Benghazi, which killed four Americans, as a "terrorist attack" that could have links to Al-Qaeda.
Secretary of State Hillary Clinton meanwhile announced an official review into security at the consulate targeted during a wave of rage across the Muslim world at a film made on US soil deemed offensive to Islam.
The Obama administration had repeatedly declined over the last week to term the attack, which killed US ambassador Chris Stevens, as an act of terror, and officials said they believed it was spontaneous rather than pre-planned.
But White House spokesman Jay Carney said on Air Force One that "it is self-evident that what happened in Benghazi was a terrorist attack."
Carney referred to testimony Wednesday on Capitol Hill by US National Counterterrorism Center chief Matthew Olsen, the first government official to call the Benghazi strike terrorism.
Olsen "said, based on the information that they have now... their judgment is that it was an opportunistic attack in which elements, including possible elements of Al-Qaeda in the Maghreb, participated," Carney recalled.
"He also made clear... we have no information at this point to suggest that this is a significantly pre-planned attack."
Carney appeared to be making a semantic adjustment to terminology used by US officials to describe the Benghazi attack, rather than acting on any new information that has come to light.
The White House has said an FBI investigation into the September 11 assault will lead wherever the evidence takes it.
President Barack Obama, asked whether Al-Qaeda was involved in the Benghazi attack, said: "We don't know yet, and so we're going to continue to investigate this."
Some Republicans, including Senator John McCain, had previously demanded that the White House, which repeatedly touts Obama's record on keeping Americans safe, admit that the attack was a terrorist act.
On Thursday, McCain blasted the administration for its initial assessment, saying "it shows the gizmo level of their knowledge of this fundamental aspect of terror attacks and militant operations."
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But until Olsen's testimony, top administration spokespeople declined repeated offers from journalists to brand the assault terrorism.
The State Department is under rising scrutiny about what appears to be inadequate security for Stevens and the consulate in Benghazi before the attack.
To answer some of the questions, Clinton, holding a classified briefing for lawmakers, announced an official review of security at the mission to be chaired by veteran diplomat Thomas Pickering.
Pickering served as ambassador to the United Nations from 1989 to 1992, and US envoy to Israel and Jordan.
Clinton also rejected rumors that Stevens had said in the weeks before his death that he was on an Al-Qaeda hit list.
"I have absolutely no information or reason to believe there's any basis for that," she said.
The top US diplomat said her briefing concerned the US "security posture, before and during the events and the steps we have taken since to do everything we can with host governments to protect our people and consulates."
As political ramifications of the attack rumbled in Washington, US Deputy Secretary of State William Burns flew to Libya for a ceremony to honor Stevens and three others killed in the Benghazi attack.
Burns met Libyan Foreign Minister Ashur Ben Khayal and was also expected to see Prime Minister Mustafa Abu Shagur and Mohammed al-Megaryef, head of the national assembly.
Obama, speaking at a forum for Hispanic voters in Miami, noted that those who killed Stevens and three others should not be allowed to tar the reputation of all Libyans.
"I think it's important to understand that that's not representative of the attitudes of the Libyan people towards America," Obama said.
"We liberated that country from a dictator who had terrorized them for 40 years and Chris Stevens, the ambassador there, was one of the leaders of that process.
"So when he was killed, there were vigils in Libya but also in front of the White House expressing the deep sorrow that the Libyan people felt toward them."