"We had no actionable intelligence that an attack on our post in Benghazi was planned or imminent," Clinton told a press conference after state department talks with Mexican Foreign Minister Patricia Espinosa.
"We are taking aggressive steps to protect our staff in embassies and consulates worldwide," Clinton said, amid a wave of anti-US protests around the Middle East, North Africa and South Asia.
The US is also "reviewing our security posture at every post and augmenting it where necessary," she added, stressing that Washington was also working with host governments "to make sure they know what our security needs are".
A suicide bombing in Kabul on Tuesday added to a growing toll of people killed in the week-long violent backlash triggered by a YouTube trailer for a film mocking Islam.
Among the dead are four diplomatic staff in Libya, including ambassador Chris Stevens, killed when militants besieged the US mission in Benghazi a week ago on September 11 in a four-hour sustained attack with heavy arms.
"We will not rest until the people who orchestrated this attack are found and punished," Clinton said.
But she insisted there had been adequate security arrangements in Benghazi, and said that ahead of last week's anniversary of the September 11 2001 attacks a full evaluation of "threat streams" had been carried out.
"Let me assure you that our security in Benghazi included a unit of host government security forces, as well as a local guard force of the kind that we rely on in many places around the world," she told reporters.
It emerged on Tuesday that the state department had hired a private security firm called the Blue Mountain Group – believed to be a British company which says on its website it has a heritage "gained from many years' service in UK Special Forces" – to hire local Libyan staff in Benghazi.
State department spokesperson Victoria Nuland admitted she had made a mistake when she said last week the US had not signed any private security contracts with firms to guard the Benghazi mission.
"The external security, external armed security, as we have been saying, outside of the perimeter, was fully handled by the Libyan side," Nuland told reporters on Tuesday.
"There was a group called Blue Mountain Group, which is a private security company with permits to operate in Libya. They were hired to provide local Libyan guards who operated inside the gate doing things like operating the security access equipment, screening the cars, that kind of thing."
It was unclear whether any of the local staff had been killed in Tuesday's attack, she said, nor whether the state department had worked with the company in other countries.
The FBI has launched an investigation inside Libya and Clinton said the US was working with the Libyan government, which is leading its own inquiry, "so we can be assured that we have found who murdered our four colleagues and under what circumstances".
People also had to look at the events strategically. "In a lot of places where protests have turned violent, we are seeing the hand of extremists who are trying to exploit people's inflamed passions for their own agendas."
But she said most people in the Arab Spring nations were seeking to build a better future.
"This is part of a larger debate that is going on inside of these societies," she said, highlighting how moderates had won the elections in Libya after the fall of Libyan leader Muammar Gaddafi. – Sapa-AFP