Republican lawmakers compared Barack Obama's White House to the secretive Nixon administration on Thursday, denouncing its response to the 2012 attack on a US mission in Benghazi, Libya.

On Wednesday a conservative group published a White House email it had obtained after a legal challenge and which critics say shows an attempt to put a political spin on the deadly assault.

In the mail, Obama's deputy national security advisor Ben Rhodes tells Susan Rice -- then the US envoy to the UN -- to blame the attack on local anger in Benghazi over an anti-Muslim Internet video.

It has since become clear the September 11, 2012 attack on the mission -- which cost the lives of four Americans, including Ambassador Chris Stevens -- was planned by armed militants.

In the email, obtained by Judicial Watch and dated September 14, 2012, Rhodes laid out talking points and goals for Rice in her planned appearances on several major US talk shows the next day.

She was asked "to underscore that these protests are rooted in an Internet video, not a broader failure of policy."

White House spokesman Jay Carney said Wednesday after the Rhodes email was first made public that it referred to protests in the Arab world as a whole, and not specifically to the Benghazi attack.

But the attack and Rice's media appearances took place at the height of Obama's successful re-election campaign -- a campaign in which he made great play of having put Al-Qaeda on the back foot.

Republican lawmakers accuse the White House of having tried to cover-up the organized nature of the attack, carried out by Libyan Islamist extremists with suspected Al-Qaeda ties or sympathies.

The administration, in part through Rice's early public statements, said the attack was provoked by an anti-Islamic video posted on YouTube. But this version of events was quickly proven false.

Congress launched hearings and thousands of documents were turned over, but the Rhodes email from Rhodes had not been revealed, despite requests from the House of Representatives.

"It is disturbing and perhaps criminal that these documents, that documents like these were hidden by the Obama Administration," said Representative Darrell Issa, who chairs a House commission charged with overseeing and auditing the White House.

"The American people have learned that you cannot believe what the White House says, you cannot believe what the spokespeople say, and you cannot believe what the President says," Issa said.

"The facts are coming out, that in fact this administration has knowingly withheld documents pursuant to congressional subpoenas in violation of any reasonable transparency or historical precedent, at least since Richard Milhous Nixon."

Nixon resigned as president in disgrace in 1974 after the Watergate scandal broke over illegal activities, and attempted cover-ups, by the administration.