WASHINGTON -- U.S. government personnel are searching for al-Qaida fighters in the rugged tribal regions of northwest Pakistan, and the American military is set to send in troops to join the hunt, officials said Thursday.

The United States and Pakistan reached an agreement several weeks ago allowing American military operations on Pakistani soil, U.S. officials said. This will allow Americans to hunt in a suspected al-Qaida haven previously closed to them -- tribal areas that are traditional rallying points for fighters fleeing Afghanistan.

The operations carry considerable risk, physically for the Americans and politically for Pakistani President Pervez Musharraf, who faces a referendum on Tuesday to extend his term as president for five years. He has defied strong anti-American sentiment to support President Bush in countering terrorism.

Pakistan's tribal belt is ruled by deeply conservative and fiercely independent tribesmen who swear little allegiance to anyone but their 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.

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

"No U.S. personnel are present in Pakistan's tribal areas searching for al-Qaida 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-Qaida.

Anti-American areas

The Pakistani army moves softly in this region. Large madrassas -- 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 Osama bin Laden," referring to the Taliban and al-Qaida leaders.

The tribal areas are just over the border from Afghanistan's Paktia and Paktika provinces, where bin Laden has been and where much of al-Qaida was based before the war in Afghanistan. U.S., Canadian and British forces have conducted operations in those provinces.

The porous border has little meaning to locals and al-Qaida, but holds substantial political significance for the United States and Pakistan. Thousands of Pakistani troops are tied up facing India at its eastern reaches, but 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 area.

U.S. soldiers occasionally pursue al-Qaida fighters across the border into Pakistan, a senior U.S. official told the AP.

But this is separate from the operation being prepared for Pakistan itself, he said, speaking on condition of anonymity.

Asked about reports 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 of the officials said.

Some of those terms were unclear, including whether the United States could use its substantial air power in the region.

Because of Pakistani support, the United States also 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 officers took part in the urban raids that led to last month's capture of Abu Zubaydah, Osama bin Laden's top field commander.

Previously, Pentagon officials had indicated that they thought it was 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, had been to coordinate and consult with the Pakistani military in pursuing al-Qaida 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.

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Associated Press writers John J. Lumpkin in Washington and Kathy Gannon in Islamabad contributed to this report.