WASHINGTON (AFP) — North Vietnamese made hoax calls to get the US military to bomb its own units during the Vietnam War, according to declassified information that also confirmed US officials faked an incident to escalate the war.

The report was released by the National Security Agency, responsible for much of the United States' codebreaking and eavesdropping work, in response to a "mandatory declassification" request, the Federation of American Scientists (FAS) said Monday.

From the first intercepted cable -- a 1945 message from Vietnamese leader Ho Chi Minh to his Russian counterpart Joseph Stalin -- to the final evacuation of US spies from Saigon, the 500-page report retold Vietnam War history from the perspective of "signals intelligence", the group said in a statement.

During the war, North Vietnamese intelligence units sometimes succeeded in penetrating US communications systems, and they could monitor American message traffic from within, according to the report "Spartans in Darkness".

On several occasions "the communists were able, by communicating on Allied radio nets, to call in Allied artillery or air strikes on American units", it said.

"That's something I have never heard before," Steven Aftergood, director of the FAS project on government secrecy, told AFP.

But he said that probably the "most historically significant feature" of the declassified report was the retelling of the 1964 Gulf of Tonkin incident.

That was a reported North Vietnamese attack on American destroyers that helped lead to president Lyndon Johnson's sharp escalation of American forces in Vietnam.

The author of the report "demonstrates that not only is it not true, as (then US) secretary of defense Robert McNamara told Congress, that the evidence of an attack was 'unimpeachable,' but that to the contrary, a review of the classified signals intelligence proves that 'no attack happened that night,'" FAS said in a statement.

"What this study demonstrated is that the available intelligence shows that there was no attack. It's a dramatic reversal of the historical record," Aftergood said.

"There were previous indications of this but this is the first time we have seen the complete study," he said.