William Burns, the deputy secretary of state, has acknowledged the US state department's mistakes in Libya, saying more had to be done to avoid a repeat of similar attacks in future.
"We learned some very hard and painful lessons in Benghazi," Burns told the Senate Foreign Relations Committee on Thursday. "We are already acting on them. We have to do better."
A report by the Accountability Review Board says "systematic failures" at the US State Department led to the attack at the US mission in the Libyan city of Benghazi which left four Americans dead, including an ambassador.
The unclassified report talked of "leadership and management deficiencies at senior levels" of the State Department, stressing that security was "inadequate for Benghazi and grossly inadequate to deal with the attack that took place".
The Benghazi attack, which killed ambassador Christopher Stevens and three other Americans, coincided with the 11th anniversary of the 9/11 attacks on the World Trade Centre in New York City and the Pentagon.
It was initially thought to have been prompted by a video made in the US denigrating Islam, but days later it emerged that elements linked to al-Qaeda were behind the assault.
Testifying at the first of two Congressional hearings, Burns said his boss, Hillary Clinton, was putting in place 29 recommendations made by a special review board.
Clinton, who had been scheduled to testify before the committees but cancelled after fainting and sustaining a concussion, is also creating a position to focus on diplomatic security for high-threat posts.
Clinton is under doctors' orders to rest, but legislators still want to hear her testimony.
The investigation's conclusions and the political fallout from the attack has led to the resignation of four state department officials.
"We learned some very hard and painful lessons in Benghazi," Burns told the Senate Foreign Relations Committee. "We are already acting on them. We have to do better."
Republicans have argued with the officials over whether warning signs of a deteriorating security situation were ignored and why the department did not ask Congress for money to boost security at the mission.
Benghazi was relatively lawless after the revolution that toppled Libyan leader Muammar Gaddafi.
"We made the mistaken assumption that we wouldn't become a major target," Burns said.
Many Republicans have used the Libya attack to criticise the Obama administration and its response.
The criticism has since snowballed into opposition to UN Ambassador Susan Rice as a possible candidate to succeed Clinton.
Rice, who dropped out of the race last week, blamed the attack not on terrorism but on protests against the anti-Muslim film.
But while defending herself, she explained that she had relied on talking points drawn up by intelligence agencies.