Dr Afridi finally collected blood samples from Osama bin laden's compound on April 27 which matched DNA tests from a laboratory in Washington, DC.

On May 2, US Navy Seals arrived in two helicopters, raided bin Laden's hideout, killed him and eventually buried him at sea. Peter flew out of Pakistan the same day.

Dr Afridi disappeared after the operation and went underground. He was arrested by Pakistani security agencies at the Torkham border on May 22 while trying to cross over to Afghanistan, 20 days after bin Laden had been killed.

His 33-year sentence has clearly become a thorn in already tense Pakistan-US relations. Shocked over the decision to imprison Dr Afridi for 33 years, two US Senators have asked Pakistan to pardon and release him immediately.

'It is shocking and outrageous that Shakil Afridi, the Pakistani doctor who assisted the US in the search for Osama bin Laden, has been sentenced to 33 years in prison for the crime of treason,' Senators John McCain and Carl Levin said in a joint statement.

'We call upon the Pakistani government to pardon and release Dr Afridi immediately. At a time when the US and Pakistan need more than ever to work constructively together, Afridi's continuing imprisonment and treatment as a criminal will only do further harm to US-Pakistani relations, including diminishing Congress's willingness to provide financial assistance to Pakistan,' the two Senators said.

McCain and Levin said that what Dr Afridi did was a courageous, heroic, and patriotic act, which helped locate the most wanted terrorist in the world -- a mass murderer who had the blood of many innocent Pakistanis on his hands.

Pakistan's foreign office spokesman has made it clear that Dr Afridi has been dealt with according to the laws of the land and the sentence awarded to him is an internal matter of Pakistan.

'No foreign country,' the spokesman added, 'has any right to interfere in the internal affairs of Pakistan.'

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