US defence secretary Caspar Weinberger was left stunned by Israel's air strike on an Iraqi nuclear reactor in 1981, according to secret British files released Friday.

Despite the close ties between Israel and the United States, Weinberger was taken aback by the strike on the unfinished Osirak reactor outside Baghdad, papers released after 30 years in Britain's National Archives showed.

Israeli prime minister Menachem Begin ordered the June 1981 raid amid fears that Iraqi dictator Saddam Hussein was trying to build a nuclear weapon.

Sir Nicholas Henderson, Britain's ambassador to Washington, was with Weinberger as the news came in.

"Weinberger says that he thinks Begin must have taken leave of his senses. He is much disturbed by the Israeli reaction and possible consequences," Henderson cabled London.

Meanwhile Sir Stephen Egerton, Britain's ambassador in Baghdad, said the Iraqis had been just as surprised when Israeli fighter jets appeared in their skies.

"The diplomatic corps had a ringside view of the belated ack-ack and missile reactions to the raid when we were gathered for the Italian national day reception on the Bund," he wrote.

"The raiders had gone but the fireworks were spectacular."