Lawmakers divided along party lines as US officials Thursday urged more funding to protect diplomats in hearings into the Benghazi attack marked by the absence of Secretary of State Hillary Clinton.

House and Senate committees probed how to prevent any more attacks such as the September assault on the US mission in Benghazi after an inquiry faulted failures in the State Department for "grossly inadequate" security there.

"We clearly fell down on the job with regards to Benghazi," Deputy Secretary Bill Burns admitted to the House foreign affairs committee.

But he stressed the department accepted all 29 recommendations by the five-strong Accountability Review Board (ARB) and had already begun to fix some of the problems highlighted in its damning 39-page report.

The ARB also called for $2.3 billion in extra funding over the next 10 years to fortify and improve some of the 275 US diplomatic outposts around the world.

"It's no understatement that our diplomats are on the front lines of the world's most dangerous places," Democratic Senator John Kerry, chairman of the Senate Foreign Relations committee, said.

The changes sweeping the Arab world have shifted the ground rules, meaning that in many places embassies can no longer rely on local police and armed forces to ensure their security, as has been the tradition for two centuries.

But Kerry said ambassador Chris Stevens, who died along with three other Americans in the Benghazi attack, would have been one of the first to stress that diplomacy cannot be effective if done from behind fortress walls.

"There will always be a tension between the diplomatic imperative to get outside the wire and the security standards that require our diplomats to work behind high walls, concertina wire and full-body searches," he said.

"We do not want to concertina wire America off from the world," said Kerry, in what could be seen as a mission statement by the man widely touted to replace Clinton as the next secretary of state in 2013.

Clinton had been due to address Thursday's hearings, including a later meeting in the House, but has been ordered to rest by doctors after catching a stomach virus that caused her to faint and suffer a concussion.

Burns said the ailing top US diplomat had now "expressed her willingness" to appear before US lawmakers in mid-January.

Clinton has also called on Congress to reallocate some $1.4 billion in 2013 in funds earmarked for Iraq to help the State Department boost its security posture, the State Department said in a statement.

Some $535 million would fund an extra 225 Marines to be dispatched to high-threat posts, another $736 million will go for construction costs at embassies, and $130 million would pay for extra Department security personnel.

Retiring Republican Senator Dick Lugar, highlighted how recently the State Department budget had been "a popular target for cuts."

"Diplomacy is not a luxury. It is essential to American national security, especially in an era of terrorism. We should fund the State Department as the national security agency that it is," he insisted.

And Democratic Senator Barbara Boxer drew comparisons between the $300 million slashed from the department's budget last year, and the same amount which goes annually to funding military bands.

"We need to get our priorities straight around here," she insisted.

But Republican Senator Bob Corker fumed it was not a question of money, as there had been a special team on the ground in Tripoli whose tour had not been extended by the State Department despite concerns over security in Libya.

"The culture of the state department is one that needs to be reformed," he said.

Representative Ileana Ros-Lehtinen, chairwoman of the House committee, singled out a new global culinary diplomacy program as well as a billion dollars spent by State on global warming initiatives for ridicule.

She denounced such "misplaced priorities" saying "this money could have been used for providing diplomatic security, including hiring additional personnel and providing them with adequate equipment and training."