The United States State Department has denied having any knowledge of a letter purportedly written by Senator Thomas Graham to the then Ambassador in India Harry Barnes in 1995 about a spy in the Indian Prime Minister's Office who leaked nuclear secrets to the US.

"I'm not aware of the letter," spokesman Sean McCormack said on Wednesday. "We'll look into it."

On Tuesday, Bharatiya Janata Party leader Jaswant Singh had claimed in Parliament that Graham had written a letter to Barnes about a mole in the then prime minister P V Narasimha Rao's office.

Singh also refers to the letter in his book A Call To Honour.

But Barnes, who was the US envoy to India during 1981-85, told PTI on Wednesday he did not know any Senator by the name of Thomas Graham.