Henry Kissinger gave Argentina’s military junta the green light to suppress political opposition at the start of the “dirty war” in 1976, telling the country’s foreign minister: “If there are things that have to be done, you should do them quickly,” according to newly-declassified documents published yesterday.

State department documents show the former secretary of state urged Argentina to crush the opposition just months after it seized power and before the US Congress convened to consider sanctions.

“We won’t cause you unnecessary difficulties. If you can finish before Congress gets back, the better,” Mr Kissinger told Admiral Cesar Augusto Guzzetti, the foreign minister, according to the State Department’s transcript.

Carlos Osorio, an analyst at the National Security Archive, a US pressure group which published the transcript, said it was likely to be seen by historians as “a smoking gun”.

It is likely to be seized on by Mr Kissinger’s critics who have been calling for him to face charges for abetting war crimes and human rights abuses in Cambodia, Chile and Argentina.

The Argentine junta formed a secret pact in 1976 known as the Condor Plan with other South American dictatorships in Chile, Paraguay, Bolivia, Uruguay and Brazil for the eradication of “terrorists”. According to official figures, nearly 9,000 people disappeared in Argentina alone but human rights organisations put the figure nearer to 30,000.

“The newly-revealed documents prove that as early as June 1976 Kissinger was informed of the existence of the Condor Plan,” said Horacio Verbitsky, head of the Argentine human rights group Cels in Buenos Aires.

Mr Verbitsky, who during the 1970s ran an underground news service, said Mr Kissinger made it difficult for the US embassy in Buenos Aires to pressure Argentina’s generals on human rights violations. “When US ambassador Robert Hill met with the generals to demand an end to the violence, the generals could say, your boss Kissinger knows what’s happening and he doesn’t care,” he said.

The documents include a state department transcript of a conversation between Mr Kissinger, then secretary of state in the Ford administration and Mr Guzzetti, on October 7 1976, six months after the Argentine military had seized power.

By that time the regime’s brutality had become clear. Mr Hill sent repeated notes to Washington, describing the abuses and his attempts to get the junta led by President Jorge Videla to stop the “disappearances” of its leftwing opponents.

But when Mr Guzzetti raised the issue at the October 1976 meeting at the Waldorf Astoria hotel in New York, Mr Kissinger told him: “Look, our basic attitude is that we would like you to succeed. I have an old-fashioned view that friends ought to be supported. What is not understood in the United States is that you have a civil war. We read about human rights problems but not the context. The quicker you succeed the better.

“If there are things that have to be done, you should do them quickly. But you must get back quickly to normal procedures.”

Mr Kissinger remains an influential voice on foreign affairs in Washington. His office at his lobbying firm, Kissinger Associates, did not return calls seeking comment yesterday.

William Rogers, a former state department official who attended the Guzzetti meeting and is now vice-chairman of Kissinger Associates told the Associated Press: “It’s a canard ... The idea that he would tell another country to violate human rights quickly or slowly or under any circumstances is preposterous.”

The National Security Archive, which campaigns for government transparency and pursues the publication of classified documents, had received the transcript of the Guzzetti meeting in February, in response to a request under the Freedom of Information Act. However, the key passages in the conversation had been blacked out. The organisation appealed and the deleted sections were reinstated.

According to another state department document, Mr Hill said the Argentine generals had returned from their meeting “euphoric”.

In a memo from a top Kissinger aide at the state department, Mr Hill was assured that Mr Guzzetti had “heard only what he wanted to hear”, and that he had in fact been told “the USG [US government] regards most seriously Argentina’s international commitments to protect and promote fundamental human rights..”

Mr Hill later found he had been lied to, and confided his disgust to Patricia Derian, a former assistant secretary of state for human rights who visited him in Buenos Aires in 1977.

“He said Kissinger had admitted to him exactly what has now come out in the documents,” Ms Derian told the Guardian

“... Kissinger has not been held to account for it. He’s only been embarrassed. He has people talk for him and say he’s misunderstood ... It’s baloney,” she said.