The army said Saturday that it has massed 15,000 troops for a major assault on Islamic militants in a scenic northern valley, whose fall has raised concern about Pakistan's ability to withstand rising extremism.

Security forces have been fighting in the Swat Valley, a former tourist destination just 100 miles from the capital, since July, when a bloody army raid on a radical mosque in Islamabad sparked a wave of militant violence.

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Foreign fighters allegedly have joined the armed followers of Maulana Fazlullah, a pro-Taliban cleric in the valley, amplifying Western fears that swaths of Pakistan near the Afghan border offer an increasingly safe haven for Al Qaeda.

Washington is expressing concern about rising violence in Pakistan, where well over 1,000 security forces, civilians and militants have died in the past five months.

A senior Pakistani commander said Saturday that the army had recorded 28 homicide attacks in that period.

"It's not Iraq, but it is getting worse," Michael Vickers, the Pentagon's assistant secretary of defense for special operations and low-intensity conflict, told The Associated Press on Friday in Washington. "We would always like them (Pakistani authorities) to do more given the importance of the problem," he said. "They're certainly doing a lot."

The army said Saturday that troops backed by helicopter gunships and artillery were attacking militants to push them back into the mountains overlooking the Karakoram Highway, Pakistan's vital overland route to China.

Between 35 and 40 rebels were killed in that push on Friday, it said in a statement. That raised the number of militants killed this week to over 100, according to army reports.

A police official said some civilians had died after shells struck their homes. The official spoke on condition of anonymity because he is not allowed to address the media, but gave no further details.

Sirajuddin, a Fazlullah spokesman who goes by one name, confirmed that the military had intensified attacks but said his forces had suffered only "some" casualties.

"Our mujahedeen are still in a strong position, and God willing we will defeat the enemy," he told AP by telephone. He accused the army of killing civilians by shelling residential areas.

The army says it is focusing its fire on mountain top positions and holding back when it thinks civilians could be endangered. It said the militants were putting up stiff resistance.

Four government troops have died in the fighting there this week, said army spokesman Maj. Gen. Waheed Arshad. Two soldiers were wounded Friday when militants attacked their convoy with hand grenades, an army statement said.

In another burst of violence, police said at least 30 people died in clashes between Sunni and Shiite Muslims in Parachinar, a town near the Afghan border.

During a rare media briefing at the army's headquarters, a senior commander gave details of the threat cited by President Gen. Pervez Musharraf when he imposed a state of emergency on Nov. 3.

Gen. Ahmed Shuja Pasha said militants from Afghanistan as well as the lawless Pakistani border regions of Waziristan and Bajur had reinforced the followers of Fazlullah in Swat.

Pasha, director general of military operations, said the army had assembled about 15,000 troops in Mingora, the valley's main town and would launch its main offensive within days.

He said they planned to push the militants back into the rugged Piochar side valley where they had established bases.

"We will bottle up as many of them as possible and then eliminate them," Pasha said. "This is our killing ground."

Pasha said the enemy comprised about 500 fighters and included a core of several dozen hardened militants — ethnic Uzbeks from Waziristan and others from Afghanistan. He accused the outsiders of beheading civilians and captured security forces. He said the army had evidence that they were funded from abroad, but declined to give details.

Arshad said members of a banned Sunni Muslim extremist group, Lashkar-e-Jhangvi, had also rallied to Fazlullah's cause.

Pasha said the local population in Swat, whose mountain scenery had begun to draw Pakistani and foreign tourists and persuaded investors to build several upscale hotels, was moderate but powerless to resist in the face of deadly intimidation by the militants.

He said the army could clear the region in three days were it not for the need to avoid heavy shelling in residential areas and that it hoped to reopen the area to the public by the end of December.

Fazlullah has used a pirate FM radio station to build up a following in the region and this summer urged his followers to wage holy war against the authorities in order to bring the area under Islamic law.

Pasha said Pakistan had asked the U.S. for help in blocking the broadcasts from a mobile transmitter but had yet to find a way.