PESHAWAR, Pakistan - Pakistani paramilitary troops sent to protect government buildings and bridges in a district recently infiltrated by the Taliban came under fire Thursday by gunmen who killed a police officer, authorities said.

The assault in Buner district is likely to heighten concern about the viability of a government-backed peace deal that imposes Islamic law in a large segment of the country's northwest in exchange for peace with Taliban militants in the neighboring Swat Valley.

In recent days, the valley's militants have entered Buner in large numbers -- establishing checkpoints, patrolling roads and spreading fear in an area some 97 kilometres from Islamabad. Their movement has bolstered critics' claims that the deal would merely embolden the militants to spread their reign to other parts of the province bordering Afghanistan.

The U.S. has become one of the deal's foremost critics.

"I think the Pakistani government is basically abdicating to the Taliban and the extremists," U.S. Secretary of State Hillary Clinton told lawmakers in a hearing Wednesday in Washington.

Six Frontier Constabulary platoons arrived in Buner on Wednesday, said Syed Mohammed Javed, a government official who oversees the area covered by the peace deal. He would not say if the deployment was in direct response to the Taliban presence, but said tribal elders were meeting to assess the overall situation.

Javed did not specify the number of troops involved, but a platoon typically has 30 to 50 members.

On Thursday, gunmen opened fire on a security convoy that included some of the Frontier Constabulary. The gunfire killed an escorting police officer and wounded another in the Totalai area, said Hukam Khan, an area police official.

He refused to speculate on the identities of the gunmen.

Maj. Gen. Athar Abbas, the Pakistan Army's chief spokesman, insisted the situation in Buner was not as dire as some have portrayed -- saying militants were in control of less than 25 per cent of the district, mostly its north.

"We are fully aware of the situation," Abbas said. "The other side has been informed to move these people out of this area."

Pakistan's army has thousands of troops in Swat but none in Buner. Many police and government officials in Buner appear to have either fled or are keeping a low profile.

A police official, speaking anonymously out of fear, told the AP that the militants in Buner were broadcasting sermons by radio about Islam and are warning barbers to stop shaving men's beards.

The militants have established a base in the village of Sultanwas and have set up positions in the nearby hills, the official said.

Istiqbal Khan, a lawmaker from Buner, said the insurgents have established checkpoints and patrols.

President Asif Ali Zardari signed off on the peace pact last week in hopes of calming Swat, where some two years of clashes between the Taliban and security forces have killed hundreds and displaced up to a third of the one-time tourist haven's 1.5 million residents.

The cease-fire agreement with militants covers Swat and other districts in the Malakand Division, an area of about 25,900 square kilometres near the Afghan border and the tribal areas where al Qaeda and the Taliban have strongholds.

Supporters say the deal will let the government gradually reassert control by taking away the militants' rallying cry for Islamic law.

Also Thursday, dozens of militants armed with guns and gasoline bombs attacked a truck terminal elsewhere in northwest Pakistan, burning five tanker trucks carrying fuel to NATO troops in Afghanistan, police said.

Mounting assaults on the critical supply line through the famed Khyber Pass are adding to worries that Pakistan is losing its grip on the northwest.

Gunmen attacked the truck depot near the city of Peshawar before dawn on Thursday, hurling gasoline bombs that set fire to the five tankers, said Abdul Khan, a local police official.

Security guards fled, and the assailants escaped before police arrived, Khan said.

NATO and the U.S. military insist that their losses on the transport route remain minimal and have had no impact on their expanding operations in Afghanistan. However, they have been seeking alternative routes through Central Asia.

The chairman of the U.S. joint chiefs of staff, Adm. Mike Mullen, was in Pakistan on Thursday for talks with Pakistani officials, the U.S. Embassy said.