WASHINGTON: As bad as President Donald Trump describes U.S.-Pakistani ties today, they can get far worse.

Over 16 years that included hundreds of deadly U.S. drone strikes, Osama bin Laden's killing on Pakistani soil and accusations Pakistan helps insurgents that kill Americans, the reluctant allies never reached one point of no return: Pakistan closing the air routes to Afghanistan.

It's an action that could all but cripple the U.S.-backed military fight against the Taliban. It could also be tantamount to Pakistan going to war with the United States.

Even if such a step is seen as unlikely by most officials and observers, Pakistan's ability to shape the destiny of America's longest war is a reminder of how much leverage the country maintains at a time Trump is suspending hundreds of millions of dollars in military assistance.

"There's some suggestion that we have all of the cards in our hands," said Richard Olson, a former U.S. ambassador to Pakistan. "But we don't. The leverage is strong on the Pakistan side as well and arguably stronger than our side."

Trump's re-commitment of U.S. forces to the fight in Afghanistan makes the stakes high for his administration.

The top U.S. diplomat for South Asia, Alice Wells, made a low-key visit to Islamabad this week, suggesting both sides want to prevent a breach in ties.

Pakistan's cooperation is needed not only to reduce violence in its northern neighbor. It's also critical to any hope of a political settlement with the Afghan Taliban after decades of conflict.

Defense Secretary Jim Mattis has said the U.S. doesn't expect Pakistan to cut off supply routes. Even so, the U.S. is seeking out alternatives, a senior administration official said, without elaborating on what those routes might be.

The Pentagon wouldn't discuss the issue, citing operational security, other than to say military planners develop "multiple supply chain contingencies" to sustain their mission.

The administration official, who wasn't authorized to comment by name and demanded anonymity, said it would be "very difficult" but not impossible for the U.S. to get military equipment into Afghanistan if the Pakistan route is shut down.

Restrictions limit what types of supplies can flow through the Northern Distribution Network in Central Asia, set up during the Obama administration amid concerns about relying solely on Pakistan.

Pakistan has cut overland access before. When a U.S. airstrike killed 24 Pakistani soldiers at the Afghan-Pakistan frontier in late 2011, months after the U.S. commando raid that killed bin Laden, Pakistan blocked border crossings into Afghanistan.

The decision sunk U.S.-Pakistani relations to a post-9/11 low point. Supply trucks that trundle across desert into Afghanistan's southern Kandahar province or into Nangarhar via the mountainous Khyber Pass ground to a halt. Hundreds of containers shipped from the U.S. or the Gulf were left stranded in the Pakistani port of Karachi until mid-2012.

For the U.S., truck and rail costs inflated by about 50 percent, said David Sedney, a former Pentagon official who organized the alternative northern routes. He said deliveries by air cost three times as much or more.

But the saga, resolved through a U.S. apology, also exposed the limits of Pakistan's leverage, Sedney said. Pakistan's own economy was hurt, notably the trucking industry. And the Afghan war effort, which was then supporting more than 70,000 U.S. troops, compared with around 16,000 now, endured.

That was perhaps the result of Pakistan never closing the air corridor into Afghanistan, which U.S. pilots call "the boulevard." It's essential for ferrying ammunition and weapons for U.S. and Afghan forces, and waging war. U.S. intelligence flights and combat missions use it when taking off from U.S. bases in the Persian Gulf or from aircraft carriers in the Indian Ocean.

Since closing Pakistan's airspace would hinder America's ability to defend its forces in Afghanistan, Olson, the former ambassador, said the U.S. might regard such action as a "casus belli," or grounds for war. Other former U.S. officials echoed that assessment.

"From what I can tell we don't actually have any serious alternative," said Daniel Markey, a South Asia expert at the John Hopkins School of Advanced International Studies.

Sedney said the Northern Distribution Network, which fell out of use after most U.S. forces were withdrawn from Afghanistan by late 2014, could be restored with astute U.S. diplomacy. Nations such as Uzbekistan, Kyrgyzstan, Tajikistan and Kazakhstan all have been used before for transporting mostly nonlethal supplies. Poor U.S. relations with Russia could make the task trickier, however. Moscow wields significant influence over these former Soviet states.

Pakistan is weighing options carefully. The suspension of around $1.2 billion in assistance and Trump's accusations of Pakistani "lies and deceit" for allowing Taliban havens have stirred anger and demands from opposition party leader Imran Khan for both land and air links to be cut.

Pakistan's ambassador in Washington, Aizaz Chaudhry, indicated such steps weren't imminent, urging greater U.S. cooperation on counterterrorism. But he warned that further downward spiraling in U.S.-Pakistani ties could create a situation in which "everything will be on table."

Chaudhry cited Pakistan's longstanding complaints that its efforts have been unappreciated, claiming that most leaders of the Haqqani network, which the U.S. hopes to eradicate, have fled to Afghanistan.

"The problem is we have a porous open border and it's like a revolving door," Chaudhry told The Associated Press. "These elements tend to come back, and travel back and forth, but there is no organized presence or safe havens inside Pakistan."— AP