WASHINGTON: Pakistan's closure of supply routes to the Afghan war is costing American taxpayers $100 million a month, said Defense Secretary Leon Panetta, as he recommended possibly setting conditions on future U.S. aid to Islamabad.

Panetta's decision to disclose what had been a closely guarded figure at the Pentagon appeared to be another sign of frustration with Pakistan and will do little to generate sympathy for that country in Congress, which is wrestling with ways to scale back the U.S. budget deficit.

Asked during a Senate budget hearing whether he would recommend halting aid to Pakistan, Panetta said: "I'd be very careful about just shutting it down."

"What I would do is look at conditions for what we expect them to do," Panetta said, without elaborating. He agreed to help write a letter to Congress with his recommendations for how to proceed with aid for the Pakistani military and government.

The comments came less than a week after Panetta, using unusually harsh language, said during a trip to Kabul that the United States was reaching the limits of its patience with Pakistan because of the safe havens it offered to insurgents fighting in neighboring Afghanistan.

The American war effort there has become far more costly, Panetta said on Wednesday, because of Pakistan's decision last November to ban trucks from carrying supplies to NATO forces in landlocked, neighboring Afghanistan.

That forced NATO to use longer, more costly routes through countries to the north of Afghanistan.

"It's very expensive because we're using the northern transit route in order to be able to draw-down our forces and also supply our forces," Panetta said.

"The amount is about ... $100 million a month because of the closure of those (Pakistani routes)."

A U.S. official, speaking on condition of anonymity, confirmed to Reuters that the $100 million figure represented the additional cost of using the northern routes rather than moving supplies on the ground through Pakistan.

Pakistan shut down the supply routes to protest a cross-border NATO air attack that killed 24 Pakistani soldiers. That strike further fanned national anger over everything from covert CIA drone strikes to the U.S. incursion into Pakistan last year to kill al Qaeda leader Osama bin Laden.

The United States on Monday announced it was withdrawing its team of negotiators from Pakistan without securing a deal to re-open the routes, publicly exposing a diplomatic stalemate and deeply strained relations between the uneasy allies.

One of the sticking points in the negotiations has been Pakistani demands that the United States apologize for the November strike, something the Pentagon has been unwilling to do. But Panetta acknowledged at the hearing that the apology wasn't the only issue.

"They're asking not only for that, but there are other elements to the negotiation that are also involved that have to be resolved," Panetta said. AGENCIES