Insurgents who attacked Pakistan's military HQ near Islamabad have taken 10 to 15 security personnel hostage, the army says.
Military officials say the attackers are surrounded inside the compound in Rawalpindi, and efforts are being made to recover the hostages safely.
Sporadic gunfire has been heard from the compound, reports from the scene say.
Six soldiers and four militants were killed in the initial assault.
The military at first said the situation was under control, but later added that up to five militants were holding the hostages inside a building within the compound.
Aleem Maqbool BBC News, Islamabad
The army's main headquarters lies within one of the most heavily secured areas in the country. To attack it in the middle of the day, and then take hostages, shows a new level of audacity on the part of the militants.
Just a few weeks ago, the government here said it was winning its fight against the militants, and that the Taliban was in disarray. The events of this week will have many questioning those claims.
Speculation that the army will soon launch a ground offensive against the Taliban in the tribal areas of South Waziristan has only left many Pakistanis bracing themselves for more violence in the cities.
Reports say senior officers were among the hostages and among those who died.
The attack comes as the Pakistani army prepares for a major operation against the Taliban.
"Eight to 10 terrorists were involved in this attack. Four of them have been killed while six of our security personnel were martyred," military spokesman Maj Gen Athar Abbas told the private Geo TV station.
He said the building had been surrounded and officials were assessing the timing of a rescue operation.
"We are trying to move with the minimum loss of life," said Gen Abbas.
Another Pakistani official was quoted by Reuters news agency as saying: "It's a hostage situation. They're demanding the release of some of their fellows."
Area sealed off
Officials said the gunmen drove up to the army compound in a white van just before midday local time (0600 GMT) on Saturday.
They took up positions, fired on the compound and threw hand grenades, security officials said.
Roads to the area were sealed off and helicopters hovered over the compound.
RECENT MILITANT ATTACKS
24 Sept - Seven pro-government tribal elders killed by militants in town of Janikhel, north-western Pakistan
26 Sept - At least 16 people killed in two suicide car bombs, in Peshawar and Bannu
5 Oct - Suicide bomber attacks UN offices in Islamabad killing five
9 Oct - At least 50 killed in suspected suicide bombing in Peshawar
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