WASHINGTON - Stepping up its pressure, the United States is asking Pakistan to release Shakil Afridi, the doctor who helped the CIA track drown Osama bin Laden , or give up $33 million from the annual aid it receives from Washington.

The bill to fund the US government binds the Obama administration to withhold $33 million, $1 million for each year of his sentence, from the funds meant for Pakistan.

The fund will be withheld “until the Secretary of State reports to Congress that Dr Shakil Afridi has been released from prison and cleared of all charges relating to the assistance provided to the United States in locating Laden.”

The CIA paid Dr Afridi to run a vaccination programme in Abbottabad in March and April 2011, as cover for an intelligence operation to establish that Bin Laden and his family were living in a large three-storey house in the town.

Dr Afridi, who was arrested in May 2011, was sentenced to 33 years in jail a year later by a court in the Khyber tribal agency for an entirely different offence - his alleged links to a banned militant group. But in August a court overturned the ruling, citing procedural errors and ordered a retrial.

Osama bin Laden was killed by US forces in Abbottabad in May 2011. His killing soured US-Pakistani relations as Pakistan saw the operation as a violation of its sovereignty.

In addition to the move to slash $33 million, Congress has also set aside clear benchmarks for Pakistan to fight terrorism. It has warned Islamabad that failure to curb terrorism could lead to the suspension of US assistance.

The suggested measures are included in a trillion-dollar spending bill Congress approved on Tuesday to fund the government for the remainder of the current fiscal year.