Pakistan's religious affairs minister has said wanted Islamic militants are in control of a besieged mosque in the capital, Islamabad.
Troops are blocking all entrances and roads to the mosque
Ejaz-ul-Haq said the militants were "hardened terrorists" holding women and children hostage inside the Red Mosque.
The Pakistani army has kept a tight grip on the mosque, also known as Lal Masjid, since fighting began last week.
The mosque's leader, Abdul Rashid Ghazi, said he and his followers would commit suicide rather than surrender.
An army commander was shot dead by students inside the mosque on Sunday.
At least 21 people have been killed since the stand-off began last Tuesday.
Abdul Rashid Ghazi said as many as 1,800 followers remained in the mosque, although this cannot be verified.
Mr ul-Haq told the BBC that between two and five of the militants were wanted in connection with "high-profile cases".
He said the militants had complete control inside the mosque, and that Abdul Rashid Ghazi was being used as a figurehead.
Mr ul-Haq said the government became aware of the militants' presence after a man killed on the first day of the siege was identified as Maqsood Ahmed, a member of Jaish-e-Mohammad, an outlawed radical Muslim organisation which has been linked to al-Qaeda.
Ahmed was wanted in connection with an assassination attempt against Pakistani Prime Minister Shaukat Aziz in the northern Pakistani town of Attock in 2004.
Abdul Rashid Ghazi has denied the presence of any banned extremist groups. He says those inside are students of his religious school and he is in charge.
Earlier, Mr ul-Haq said up to 250 militants - including foreign radicals - were leading the fighting.
'Surrender or die'
Lt Col Haroon Islam, who died on Sunday, was in charge of an operation to blow holes in the mosque compound walls to enable civilians to escape.
Water and power to the mosque have been cut off and food is said to be getting scarce.
Our correspondent says he has heard intermittent gunfire in the area and the boom of heavy weapons.
Col Islam was shot dead by students inside the mosque
An increased military presence on the streets, combined with the refusal to let a delegation of Islamic figures through to the mosque, suggests that the government is now closing the door to negotiation, our correspondent adds.
On Saturday, Pakistani President Pervez Musharraf told the students they had no option but to surrender.
"We have been patient. I want to say to the ones who have been left inside: they should come out and surrender, and if they don't, I am saying this here and now: they will be killed," he said.
More than 1,000 supporters left last week under mounting pressure from security forces, although only about 20 have left since Friday.