Pakistani soldiers in South Waziristan taking part in the assault on the Taliban
Fierce fighting is continuing for a fourth day in South Waziristan as Pakistani troops battle to gain control of the key Taliban-held town of Kotkai.
The army said it had secured the heights around Kotkai, the home to top Taliban leader Hakimullah Mehsud.
Up to 100,000 civilians have fled the conflict zone, according to the army.
The army says it has killed nearly 80 militants so far. The Taliban deny the claim. Journalists are denied access to the area and cannot verify the reports.
It seems that they [militants] want to defend this stronghold at all costs
Pakistani military official
Pakistani troops - backed by artillery, gunship helicopters and fighter jets - were reported to have briefly taken control of Kotkai in the course of fighting overnight.
But on Tuesday morning the Taliban hit back, destroying army checkpoints and killing seven soldiers, local officials said.
The officials said four Taliban militants were also killed - a claim the Taliban deny. The militants say they have yet to lose a single fighter.
"It seems that they [militants] want to defend this stronghold at all costs," a Pakistani military official was quoted as saying by the AFP news agency.
The Pakistani army said on Monday that 78 militants and nine government soldiers had been killed.
Kotkai is also the home town of senior militant commander Qari Hussain, the man reportedly responsible for training Pakistani Taliban suicide bombers who have killed hundreds of people.
US Defence Secretary Robert Gates said he was "encouraged" by the Pakistani offensive.
"I think that the terrorist attacks that have been launched inside Pakistan in recent days made clear the need to begin [to] deal with this problem.
"And so we obviously are very supporting of what the Pakistanis are doing. But it's very early yet," Mr Gates said.
Fighting is in its fourth day across much of South Waziristan. Both sides are using heavy weapons to bombard each other's positions, the BBC's Syed Shoaib Hasan in the nearby town of Dera Ismail Khan said.
FORCES IN WAZIRISTAN
Pakistan army: Two divisions totalling 28,000 soldiers
Frontier Corp: Paramilitary forces from tribal areas likely to support army
Taliban militants: Estimated between 10,000 and 20,000
Uzbek fighters supporting militants: several hundred
The army has put up checkpoints in Manzai in the west, Jandola in the east, Razmak in the north and Wana in the south-west.
The army and the Taliban are also engaged in a propaganda battle for the sympathies of the Mehsud tribe - about two-thirds of South Waziristan's population are Mehsuds.
The military says it has dropped leaflets from helicopters urging Mehsud tribesmen to rise up against the militants and to support the government offensive.
The leaflets said that the army wanted "to provide an opportunity to the Mehsud tribe to live in peace and tranquillity".
But a statement issued by the Taliban to the BBC warned Mehsud tribesmen of retaliation if they supported the government.
"We also call on the Mehsud and their leaders not to support or speak in favour of the government. If any Mehsud tribesman helps the government or speaks in their favour, strict action will be taken against them," Taliban spokesman Azam Tariq told the BBC.
There are fears that the offensive against the Mehsud group could draw into the conflict militant groups based in the Wazir tribal areas of South and North Waziristan.
In 2004 the Pakistani army suffered heavily at the hands of Wazir-affiliated militants.
The fighting in South Waziristan has caused tens of thousands of civilians to flee.
People forced to flee their homes shout and chant in support of the Taliban
The flow of refugees was unabated on Tuesday and about 8,000-10,000 people were expected to register themselves as displaced, social welfare officials told the BBC.
But officials have warned of several difficulties in setting up camps for those displaced.
A shortage of registration forms meant that new arrivals were not being offered aid and the conservative social customs of the people pouring out of the region also presented difficulties in accommodating them, officials said.
About 10,000 people from Waziristan are also reported to have fled south to the neighbouring Pakistani province of Balochistan. Reports say that until now no aid agency had been aware of their arrival in the region.
South and North Waziristan form a lethal militant belt from where insurgents have launched attacks across north-west Pakistan as well as into parts of eastern Afghanistan.
South Waziristan is considered to be the first significant sanctuary for Islamic militants outside Afghanistan since the 11 September attacks in the US. It also has numerous training camps for suicide bombers.
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