Pakistani security forces fired a series of "warning blasts" before dawn on Thursday near Islamabad's radical Red Mosque, stepping up pressure on hundreds of militant students inside to surrender, a security official said.
There were about eight explosions at intervals of several minutes, witnesses said. Some gunfire also erupted but both the blasts and gunfire stopped after about 20 minutes.
"They were warning blasts. We have not yet entered the mosque," said the official, who declined to be identified.
Hospital doctors said there were casualties during shooting overnight, raising prospects that the death toll will rise from an official tally of 16 since clashes first erupted on Tuesday.
Thursday's blasts were followed by a loudspeaker announcement calling on students inside Lal Masjid to give up, a witness said.
Liberal politicians have for months pressed President Pervez Musharraf to crack down on a pair of cleric brothers in charge of the mosque and their Taliban-style movement of thousands of religious students.
The students, some of whom have guns, had undertaken a series of provocative acts over the past six months to press for various demands including action against vice. The clerics had threatened suicide attacks if force was used against them.
In a major coup for the government, the mosque's chief cleric, Abdul Aziz, was arrested while trying to escape clad in a woman's all-enveloping burqa on Wednesday.
Aziz ran Lal Masjid with his brother, Abdul Rashid Ghazi, who was still inside the mosque, defying orders to surrender.
Speaking to Reuters by telephone on Thursday morning, Ghazi said he would take a different approach from his brother to stave off more bloodshed.
"My strategy will be a bit different from Maulana Abdul Aziz. I want this issue to resolved through dialogue," he said.
"We are not criminals. We are not terrorists that we should surrender ... we have said that we are ready for dialogue," he said, adding that conditions had been relayed earlier to Chaudhry Shujaat Hussein, leader of the ruling Pakistan Muslim League.
Should an assault be launched, a security official told Reuters, military planners know the layout, where materials were stored, and the fighting strength of the mosque's defenders.
One of the explosions overnight weakened the defenses by bringing down part of a wall of the sprawling, fortified mosque compound, and security forces also fired in teargas.
Reporters in the vicinity said security forces had ordered them away as the blasts began.
Hundreds of police and soldiers, backed by armored personnel carriers and with orders to shoot armed resisters on sight, sealed off the mosque and imposed an indefinite curfew in the neighborhood after Tuesday's clashes.
While a paramilitary officer is running operations on the ground, he has a contingent of army commandos on hand.
Up to 1,200 students have taken up a government offer of safe passage and a 5,000-rupee ($85) payment and surrendered. But, at daybreak, after the explosions and shooting, only three girls came out, paramilitary Colonel Masha Allah told Reuters.
Half of them women
Ghazi earlier told Geo TV there were 2,000 people in the mosque, half of them women.
The mosque has a long history of support for militancy but the latest trouble began in January when students, who range from teenagers to people in their 30s, occupied a library to protest against the destruction of mosques illegally built on state land.
They later kidnapped women, including some from China, who they said were involved in prostitution. They also abducted police and intimidated shops selling "obscene" Western films.
Some critics suggested the government initially saw the students' aggressive campaign as a welcome distraction from a political crisis over Musharraf's suspension of the country's top judge in March.
But heavy loss of life in an assault on the mosque would be very damaging for Musharraf in the run-up to elections this year.
The Lal Masjid movement is part of a phenomenon known as "Talibanisation" -- the spread of militant influence from remote tribal regions on the Afghan border into central areas.
A security official said a small, hard core at the mosque was unlikely to give up. Two bomb attacks on security forces on Wednesday in another part of the country killed 12 people and raised fears the mosque's militant allies were hitting back.