PESHAWAR: Pakistani authorities have sentenced a doctor accused of helping the CIA find Osama bin Laden to 33 years in jail on charges of treason, officials said, a move almost certain to further strain ties between Washington and Islamabad.

Shakil Afridi was accused of running a fake vaccination campaign, in which he collected DNA samples, that is believed to have helped the American intelligence agency track down bin Laden in a Pakistani town.

The al Qaeda chieftain was killed in a unilateral US special forces raid in the town of Abbottabad in May last year.

“Dr Shakil has been sentenced to 33 years imprisonment and a fine of 320,000 Pakistani rupees,” said Mohammad Nasir, a government official in the northwestern city of Peshawar, where the jail term will be served. He gave no further details.

Soon after his conviction, Afridi was sent to the Central Prison in Peshawar.

The court was headed by a deputy administrator responsible for Bara region.

Afridi was charged under the British-time Frontier Crimes Regulation (FCR) that unlike the Pakistan Criminal Code (CPRC), does not carry death penalty for high treason.

Afridi is the first person to be sentenced by Pakistani authorities in the bin Laden case.

US officials were strongly critical of the sentencing.

The US State Department said it saw no reason for Pakistan to detain or charge Afridi who has been accused of helping the CIA find Osama bin Laden.

“We continue to see no basis for these charges, for him being held, for any of it,” Department spokeswoman Victoria Nuland told reporters, declining specific comment on the 33-year sentence given to Shakil Afridi on charges of treason.

Key US senators demanded that Pakistan pardon Afridi, warning that the decision could put US assistance at risk.

Carl Levin and John McCain, the top senators from the two major US parties on the Senate Armed Services Committee, in a joint statement called Afridi's sentence “shocking and outrageous.””What Dr. Afridi did is the furthest thing from treason.

It was a courageous, heroic and patriotic act which helped to locate the most wanted terrorist in the world -- a mass murderer who had the blood of many innocent Pakistanis on his hands,” the two senators said.

McCain and Levin urged Pakistan to pardon and free Afridi “immediately.” "Dr. 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,” they said.

“Without commenting on specific individuals, anyone who helped the United States find bin Laden was working against al Qaeda and not against Pakistan,” said Pentagon spokesman George Little.

Bin Laden's long presence in Pakistan -- he was believed to have stayed there for years -- despite the worldwide manhunt for him raised suspicions in Washington that Pakistani intelligence officials may have sheltered him.

Pakistani officials deny this and say an intelligence gap enabled bin Laden to live here undetected.

No one has yet been charged for helping the al Qaeda leader take refuge in Pakistan.

A government commission tasked with investigating how he managed to evade capture by Pakistani authorities for so long is widely accused of being ineffective.

Afridi's imprisonment will almost certainly anger ally Washington at a sensitive time, with both sides engaged in difficult talks over re-opening Nato supply routes to US-led troops in Afghanistan.

Senior US officials had made public appeals for Pakistan, a recipient of billions of dollars in American aid, to release Afridi, detained within weeks of the raid that killed bin Laden and strained ties with Islamabad.

In January, US Defense Secretary Leon Panetta said in a television interview that Afridi and his team had been key in finding bin Laden, describing him as helpful and insisting the doctor had not committed treason or harmed Pakistan.

US Congressman Dana Rohrabacher introduced legislation in February calling for Afridi to be granted American citizenship and said it was “shameful and unforgivable that our supposed allies” charged him.

VIOLATION OF SOVEREIGNTY

The US raid that killed bin Laden in the garrison town of Abbottabad, just a few hours drive from the capital Islamabad, humiliated Pakistan's powerful military, which described the move as a violation of sovereignty.

Intelligence cooperation between the United States and Pakistan, vital for the fight against militancy, has subsequently been cut drastically.

Afridi's prison term could complicate efforts to break a deadlock in talks over the re-opening of land routes through Pakistan to US-led Nato forces in Afghanistan, which are crucial for supplies.

Pakistan closed the supply routes, also seen as vital to the planned withdrawal of most foreign troops from Afghanistan before the end of 2014, in protest against last November's killing of 24 Pakistani soldiers in a Nato air attack along the Afghan border.

Afridi's case highlighted severe tensions between Pakistan and the United States.

He was arrested soon after bin Laden was killed, and has not been publicly heard of since. Seventeen health workers who worked with Afridi on the vaccination drive were fired in March, according to termination letters seen by Reuters, which described them as having acted “against the national interest”.