Islamabad, Pakisan ? Pakistan began moving thousands of troops from the Afghan border toward India, officials and witnesses said Friday, raising tensions between the nuclear-armed neighbors and possibly undermining the U.S.-backed campaign against al-Qaida and the Taliban.

The country also announced that it was canceling all military leave in the aftermath of last month’s terror attack on the Indian financial capital of Mumbai.

India has blamed Pakistani militants for the terrifying three-day siege; Pakistan has demanded that India back this up with better evidence.

Pakistan’s latest moves were seen as a warning that it would retaliate if India launches air or missile strikes against militant targets on Pakistani soil — rather than as an indication that a fourth war was imminent between the two countries.

The reported decision threatens the critical U.S. foreign policy aim of relying on Pakistan’s military in the global battle against terrorism.

President-elect Barack Obama’s campaign promise to turn around the stalemated war in Afghanistan could be the first casualty of Pakistan’s latest moves, and the frustrated American effort to decimate al-Qaida may be the second.

Pakistan’s sudden military shift catches two administrations in mid-transition, presenting Obama with a dangerous spike in tension that his predecessor has been unable to prevent.

As President George W. Bush found out, the United States can’t wage either fight alone and can’t always persuade even well-meaning allies to set aside their own agendas and domestic politics.

To win in Afghanistan rather than merely hold ground, the United States and its allies must find a way to seal off the militants’ redoubts across the forbidding mountainous border with Pakistan. The U.S. can’t do that without Pakistan’s help, and Pakistani and Afghan militants know it.

Bush administration officials have been shuttling to New Delhi and Islamabad for weeks following the terrorist attacks in Mumbai, India, pleading with both sides not to let well-founded suspicions that the attacks originated in Pakistan become an excuse for new conflict. India and Pakistan have fought three wars, and enmity against the other has been an organizing principle for leaders of each nuclear-armed country.

If Pakistan yanks fighting forces away from what the U.S. considers the good war against terrorism in the tribal areas bordering Afghanistan, it will bear out U.S. fears of a ripple effect and show how easily militants can exploit the old rivalry.

“We hope that both sides will avoid taking steps that will unnecessarily raise tensions during these already tense times,” White House spokesman Gordon Johndroe said Friday.

“It would be really disastrous at this point if there was even a mini-war, because Pakistan is already overwhelmed with what’s going on domestically,” said Frederick Barton, a security expert at the Center for Strategic and International Studies.

“They really have lost their ability to control large parts of their country,” he said.

U.S. intelligence and military officials were still trying to determine if the reported troop movements were true, and, if so, what Pakistan’s intent may be. And they cautioned that the reports may be aimed more as a warning to India not to launch missile strikes against militant targets on its territory.