A career diplomat told Congress on Wednesday that the US mission in Benghazi in Libya had failed to meet crucial security standards, a deficiency that may have contributed to American deaths last year.

Eric Nordstrom, the regional security officer at the US Embassy in Tripoli in the months before the Benghazi attack, also said senior State Department officials were aware of security shortcomings, and it was "inexplicable" that their actions were not reviewed more thoroughly.

The testimony by Nordstrom and two other US officials -- whom Republican lawmakers have described as "whistleblowers" -- is the latest in a months-long series of hearings and studies that have put under a microscope the events of September 11, 2012, when an attack on US facilities in Benghazi left four Americans dead, including ambassador Christopher Stevens.

Republicans have hammered President Barack Obama over the attacks, accusing the White House of misleading Americans in the run up to November's presidential election by saying the attack began as a spontaneous protest over an anti-Muslim video.

The Obama administration, which later acknowledged that the attack was pre-planned, attributed the initial account to confusion about what was happening on the ground.

Nordstrom testified before the House Government Oversight Committee that senior State Department officials ordered staff to occupy the buildings despite security shortcomings in a US mission rated among the most dangerous in the world.

Of 264 overseas US diplomatic posts, "our posts in Benghazi and Tripoli were two of the 14 posts rated either high or critical in all of the threat categories on SETL (the Security Environment Threat List)," he said.

They were also "the only two facilities that met no OSPB (Overseas Security Policy Board) or SECCA (Secure Embassy Construction and Counterterrorism Act) standards."

Nordstrom said that despite the structures not meeting accepted standards, diplomatic security personnel and building operations officials told him the undersecretary of state for management "authorized occupancy of the buildings 'as is'."

He pointed to examples in Cairo, Sanaa, Tunis and Khartoum, where attacks also took place against US embassies last September, "yet resulted in only property damage versus loss of life."

Those premises had been brought up to the State Department's security standards.

At the order of then-secretary of state Hillary Clinton, State conducted an Accountability Review Board report on Benghazi.

But Nordstrom said he believed the ARB showed a "failure to review the decisions by the undersecretary for management and other senior leaders, who made critical decisions regarding all aspects of operations in Tripoli."