The entry of U.S. government personnel into the wild tribal regions of Pakistan marks the beginning of a dangerous but necessary turn, in the American view, in the hunt for Al Qaeda fighters who have taken refuge outside Afghanistan.

The American military is prepared to send in troops to join the hunt, thanks to a recent agreement between U.S. and Pakistani officials, according to several U.S. officials. U.S. personnel already are searching for remnants of the Al Qaeda terror network in rugged northwestern Pakistan; the troops would be sent in if reconnaissance were to find any fighters, said a defense official who spoke on condition of anonymity.

Tribal areas are just over the border from Afghanistan's Paktia and Paktika provinces are traditional strongholds for Usama bin Laden, the Saudi-born fugitive who heads Al Qaeda, and his followers. Many Al Qaeda camps and caves in those provinces were inherited from anti-Soviet mujahedeen fighters in the 1980s. Their backdoors always faced the Pakistani tribal areas, where the Soviets could not pursue.

The U.S., Canadian and British forces fighting in those Afghan provinces faced the same problem, unable to pursue fleeing Al Qaeda and Taliban fighters in any numbers, and had been unable to hunt Al Qaeda members in Pakistan at all.

The porous border has little meaning to locals and Al Qaeda, but it holds substantial political significance for the United States and Pakistan. President Pervez Musharraf has defied strong anti-American sentiment among the Pakistanis to support President Bush in countering terror, but the possible presence of U.S. ground forces inside Pakistan could serve to further heighten tensions against Musharraf.

The Pakistan army treads lightly in this region and has been unable to police its border alone. Thousands of Pakistani troops also are tied up facing India at its eastern reaches, and former U.S. officials with experience in the region say even hundreds of thousands of troops would be unable to plug every trail in the mountainous northwest.

Pakistan's tribal belt is ruled by deeply conservative, fiercely independent tribesmen who swear little allegiance to anyone but tribal elders and to laws laid out by tradition and the tenets of Islam. Tribesmen who live in high-walled compounds have warned against U.S. soldiers on their territory.

Large religious schools flourish, and fiercely anti-American and pro-Taliban religious parties have large followings.

The parties' flags fly from rooftops, above slogans scribbled on walls that say "death to America" and "Long live Mullah Omar and Usama bin Laden," referring to the Taliban and Al Qaeda leaders.

Publicly, Islamabad denied any knowledge of U.S. operations.

"No U.S. personnel are present in Pakistan's tribal areas searching for Al Qaeda men," Aziz Ahmad Khan, spokesman for the Pakistani Foreign Ministry, told The Associated Press.

However, Pakistani intelligence and Interior Ministry sources confirmed that civilian U.S. officials, with the help of Pakistani authorities, are quietly working in the areas to trace the remnants of Al Qaeda.

U.S. forces occasionally have pursued Al Qaeda fighters across the border into Pakistan, a senior U.S. official told the AP. This is separate from the operation being prepared for Pakistan itself, the official said, speaking on condition of anonymity.

Asked about reports that U.S. military attacks have begun, the official said, "I can't say we've gotten to that point yet."

The Bush administration and Pakistan worked out rules of engagement several weeks ago, U.S. officials said. The Musharraf government has stood by them despite reports of wavering, one said.

Some of the terms were unclear, including whether the United States could conduct airstrikes against Al Qaeda targets that emerge on Pakistani soil.

Because of Pakistani support, the United States has established a substantial presence in parts of Pakistan some distance from the Afghan border, including at several military bases used to support operations in Afghanistan. In addition, FBI agents and CIA operatives took part in the urban raids that led to last month's capture of Abu Zubaydah, Usama bin Laden's top field commander.

Previously, Pentagon officials had indicated they thought it improbable 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, had been to coordinate and consult with the Pakistan military in pursuing Al Qaeda fugitives but to let Pakistan carry out most of the operations.

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