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VIENNA, Nov 21 (Reuters) - The U.S. ambassador to the U.N.'s nuclear watchdog said on Friday there was "no rational basis" to assume Iran was not still deceiving the body about the full extent of its atomic programme.

"So much of what (Iran) has said in the past year about its nuclear programme has turned out to be false that there is no rational basis simply to assume the contrary now," U.S. ambassador Kenneth Brill said in the written text of a speech he made to the U.N. watchdog's governing board.

Brill also criticised the U.N. International Atomic Energy Agency (IAEA) for stating in a recent report on Iran's nuclear programme that there was "no evidence" Iran had a secret weapons programme, as Washington alleges.

"This misleading phrasing moved both (U.S) government officials and academic experts across the political spectrum to expressions of disbelief that the institution charged with... scrutinizing nuclear proliferation risks was dismissing important facts that had been disclosed by its own investigations," he said.

The IAEA report said Iran had admitted to concealing a uranium enrichment programme for 18 years and had secretly reprocessed a small amount of plutonium, useable in an atomic weapon.

Brill said that the IAEA should have used the words "no proof" instead of "no evidence".

"It will take time to overcome the damage caused to the (IAEA's) credibility by this highly unfortunate and misleading 'no evidence' turn of phrase," he said.

Although the IAEA said it had not found any evidence of a secret weapons programme, it said the jury was still out as to whether one existed.