A long-awaited inquiry into a deadly militant attack on the US mission in the Libyan city of Benghazi late Tuesday slammed State Department security arrangements there as "grossly inadequate."

But the months-long probe also found there had been "no immediate, specific" intelligence of a threat against the mission, which was overrun on September 11 by dozens of heavily armed militants who killed four Americans.

"Systemic failures and leadership and management deficiencies at senior levels within two bureaus of the State Department resulted in a Special Mission security posture that was inadequate for Benghazi and grossly inadequate to deal with the attack that took place," the report said.

The Accountability Review Board (ARB) also concluded "there was no protest prior to the attacks, which were unanticipated in their scale and intensity."

The attacks, in which the consulate and a nearby annex were targeted, have become fiercely politicized, with Republicans skewering the administration for security failings as well as a possible cover-up over Al-Qaeda's role.

The US envoy to the United Nations, Susan Rice, came under relentless Republican fire for saying days after the assault that, according to the best intelligence, it was triggered by a "spontaneous" protest outside the mission.

Rice has since been forced to pull out of the running to replace US Secretary of State Hillary Clinton, who steps down early in 2013.

A report on the inquiry is available below:

In the unclassified section of their report, the five-strong board said they believed every effort had been made to rescue ambassador Chris Stevens, who died in the attack -- the first US envoy killed on duty in three decades.

Clinton said she accepted "every one" of the 29 recommendations made by the ARB, which has spent the last three months investigating the events.

She also said the State Department was working with the Pentagon to "dispatch hundreds of additional Marine Security Guards to bolster our posts" and was aiming to train up more diplomatic security personnel.

The report provided "a clear-eyed look at the serious, systemic challenges that we have already begun to fix," Clinton said in a letter to lawmakers, adding that while everyone at State had a duty to ensure diplomats' safety, "most of all it is my responsibility as secretary of state."

Clinton also backed the report's findings urging Congress to support moves to realign the department's 2013 budget request to help reinforce its diplomatic outposts, in her letter provided to journalists.