US senators scandalised by Pakistan's jailing of a doctor for helping the CIA track down Osama bin Laden have voted to cut aid to Islamabad by $33mn - one million for each year in the doctor's sentence.

"It's arbitrary, but the hope is that Pakistan will realise we are serious," said Senator Richard Durbin after the unanimous 30-0 vote by the Senate Appropriations Committee on Thursday.

"It's outrageous that they [the Pakistanis] would say a man who helped us find Osama bin Laden is a traitor," said Durbin, the senate's number two Democrat.

The Senate Armed Services Committee later passed a measure that could lead to even deeper cuts in aid.

The sentencing on Wednesday of Dr Shakil Afridi for 33 years on treason charges added to US frustrations with Pakistan over what Washington sees as its reluctance to help combat Islamist militants fighting the Afghan government and the closure of supply routes to NATO troops in Afghanistan.

Hillary Clinton, the US secretary of state, called the jailing of the doctor "unjust and unwarranted" and vowed to continue to press the case with Islamabad. "The United States does not believe there is any basis for holding Dr. Afridi."  

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 last year.

The al-Qaeda leader was killed in the town of Abbottabad a year ago in a unilateral US special forces raid that heavily damaged ties between Islamabad and Washington. Since then, there have been growing calls in the US congress to cut off some or all of US aid.

Senator John McCain, top Republican on the Armed Services Committee, said legislators had agreed to withhold certain military aid for Pakistan until the defense secretary certifies that Pakistan is not detaining people like Afridi.

"All of us are outraged at the imprisonment and sentencing of some 33 years - virtually a death sentence - to the doctor in Pakistan who was instrumental ... in the removal of Osama bin Laden," McCain said, adding that Afridi was innocent of any wrongdoing. "That has frankly outraged all of us."

The Senate Appropriations Committee's action docking Pakistan's aid came after a subcommittee earlier in the week slashed assistance to Islamabad - and warned it would withhold even more cash if Pakistan does not reopen supply routes for NATO soldiers in neighboring Afghanistan.

Pakistan has been one of the leading recipients of US foreign aid in recent years. Even after the cuts voted this week it still would receive about $1bn in fiscal 2013, if the full Senate and House of Representatives approve.