Under an agreement worked out with Pakistan weeks ago, American troops are set to go after Al Qaeda militants in the South Asian country, four well-placed U.S. officials said Thursday.

The covert operation will extend the U.S. conflict against Usama bin Laden's terror operation in Afghanistan across the border to Pakistan as President Pervez Musharraf validates his support for the Bush administration.

"We have a shared concern with the Pakistanis," one of the officials told The Associated Press. But asked about reports the operation has begun, the official said, "I can't say we've gotten to that point yet."

Defense Secretary Donald H. Rumsfeld, traveling to Central Asia, would not comment to reporters on his plane about what was happening in Pakistan.

"We do not characterize what other countries are doing or what we are doing in other countries," he said. "Pakistan has, from the outset, been enormously helpful and cooperative in the global war on terror."

He acknowledged that U.S. agencies outside the Defense Department were involved in the apprehension in Pakistan last month of Abu Zubaydah, bin Laden's senior lieutenant, who is under interrogation at an undisclosed location.

The administration and Pakistan worked out rules of engagement several weeks ago. This established terms for the operation, and the Musharraf government has stood by them despite reports of wavering, a second U.S. official said.

"The operation has not yet begun," said a third U.S. official, also speaking on condition of anonymity. He described as premature a newspaper account that U.S. military units had participated in attacks on suspected Al Qaeda hideouts.

Also, the official said, American soldiers had not been wounded, as The Washington Post reported Thursday.

A fourth official said U.S. forces in Afghanistan had chased Al Qaeda fighters across the border sporadically over the past few weeks, but that was separate from the covert operation being planned.

Covert U.S. soldiers are searching for pockets of militants along the border region, the Post said.

U.S. troops based on the Afghanistan side of the mountainous frontier have been attacked several times a week over the last month and have been in several fire fights with Al Qaeda militants, the Post said, citing unidentified U.S. military officials.

U.S. forces have found only small pockets of Al Qaeda forces since the end of a weeklong ground and air assault in the Shah-e-Kot valley south of Kabul, the Afghan capital. Since then, the military has been quiet on whether U.S. forces are operating in Pakistan, where many Al Qaeda fighters are believed to have fled.

In the Afghan regions, members of the U.S. special forces and Delta Force have been deliberately exposing themselves to attack to draw out the small pockets of Al Qaeda and Taliban fighters believed to be hiding in the border area, the Post said, again citing military officials.

U.S. officials earlier Wednesday had said the Bush administration was considering sending U.S. advisers to work with Pakistani troops in the pursuit of Al Qaeda and Taliban fighters in Pakistan.

Pentagon spokeswoman Victoria Clarke said Pakistan had been very helpful in the war on terrorism.

"The cooperation continues to get better and better all the time," she said.

At issue is a strategy to deal with hundreds of Al Qaeda and Taliban militants who are believed to have taken refuge in a lawless region near the Afghan border.

One official said Pakistan was reluctant to acknowledge contacts with the United States about joint military cooperation in tracking down terrorists out of concern for public opinion.

He noted that Pakistani authorities themselves traditionally have been reluctant to engage in law enforcement activities in the area, preferring to leave policing to local tribal authorities.

Pentagon officials have indicated for months that they think it is unlikely that Pakistan would agree to joint military operations in pursuit of suspected terrorists inside Pakistan, and some have said they believe it would be unwise because of a likely political backlash.

The approach taken by Gen. Tommy Franks, the U.S. commander of the war in Afghanistan, has been to coordinate and consult with the Pakistani military in pursuing Al Qaeda fugitives, but to let the Pakistani authorities carry out the operations.

The main U.S. role has been in providing intelligence and law enforcement support, rather than direct military involvement.