WASHINGTON (AFP) - Republican lawmakers on Tuesday released a report on the deadly 2012 assault on the US mission in Benghazi, accusing then secretary of state Hillary Clinton of underestimating and then misrepresenting the extremist threat in Libya.

As Washington insiders pored over the minutiae of an 800-page document that was two years and seven million taxpayer dollars in the making, there was no sign of the kind of bombshell revelation that would fatally wound Clinton's campaign for the White House.

But every hour her team spends responding to the charges - including details of an abandoned bid to deploy troops to protect the Libyan outpost, and of behind-the-scenes meetings in Washington - is a distraction from her effort to build an unassailable lead over Donald Trump.

Clinton's Democratic supporters on the Congressional panel probing the attack and its political aftermath denounced their colleagues' response, and the State Department said it provides few new facts about the September 2012 tragedy not already covered by multiple former investigations.

But there is some material in the report that will feed the narrative of those who believe the US administration was too slow to react to mounting danger in Libya and too quick to blame unconnected protests about a US-made video.

This in turn will further raise the temperature in an already bitter campaign to succeed US President Barack Obama in the White House - a campaign in which Clinton herself is now the Democratic frontrunner and leading the Republican billionaire Trump in opinion polls.

Republican Congressman Trey Gowdy, who chaired the select committee set up to probe the attack, released the report with a tribute to the four Americans who died: Ambassador Chris Stevens, State Department officer Sean Smith and CIA security contractors Glen Doherty and Tyrone Woods.

"Now, I simply ask the American people to read this report for themselves, look at the evidence we have collected, and reach their own conclusions," he said.

Some of Gowdy's Republican colleagues had some ideas about what voters' conclusions should be.

"We expect our government to make every effort to save the lives of Americans who serve in harm's way. That did not happen in Benghazi," said Congressman Mike Pompeo.

"Politics were put ahead of the lives of Americans, and while the administration had made excuses and blamed the challenges posed by time and distance, the truth is that they did not try."

Such rhetoric did not impress Clinton's camp, who noted that the Democratic members of the committee had shunned the report and accused the Republicans of stirring controversy for purely political ends.

"Far from honouring the four brave Americans who died, the Benghazi Committee has been a partisan sham since its start," Clinton campaign press secretary Brian Fallon wrote in a tweet.

In its initial response, the State Department also paid tribute to the four slain Americans.

Spokesman Mark Toner said: "We honour them by working every day to internalise the lessons of Benghazi."

"The essential facts surrounding the 2012 attacks in Benghazi have been known for some time," Toner added, citing two independent government reviews and seven previous Congressional committees.

The reports' authors highlighted details that, while not proving the widespread conspiracy theory that Obama's White House or Clinton's State Department delayed a possible rescue, do support the view that political considerations were in play from an early stage.

The attack by organised Libyan Islamist extremists on Stevens' exposed US compound in the eastern city came as Obama was engaged in a tough re-election battle against Republican challenger Mitt Romney.

In the days immediately after the shocking assault, which happened on the 11th anniversary of the Sept 11, 2001 attacks, some US officials wrongly claimed it had sprung out of a protest against an anti-Muslim YouTube video.

The Republican report provides evidence that many within the administration knew early on that it had in fact been a planned "terrorist attack," even as others were painting it as an isolated response to a specific provocation.

The report lays out an account of a White House meeting held when Stevens was already missing and the fighting still underway in which officials agreed "action items" related to the YouTube video.

It also describes, citing testimony from participants, how an elite Marine team based in Spain changed in and out of its uniforms four times while waiting to deploy, while officials mulled whether US insignia would offend Libyans.

Previous reports have concluded that no US units could have deployed in time to save the diplomats or the CIA team posted nearby.