Islamabad - Pakistan's top security official on Monday pledged a fight to the end as the military pushed forward against Taliban rebels in the Swat valley amid a mass civilian exodus.

Security forces backed by aircraft launched a full-scale onslaught against the militants Friday to reclaim large swaths of mountain territory, just 140 kilometers north-west of the capital, Islamabad.

"Let me make it clear that the military operation will continue until the last militant is flushed out," Interior Minister Rehman Malik told reporters in Islamabad.

Prime Minister Yousuf Raza Gilani called the offensive "a fight for the country's survival," as the US government welcomed the army action against the Taliban insurgency, which it said threatened "the very existence" of the lone Islamic nuclear power.

Malik said up to 700 militants had been killed in Swat and its nearby districts while only 20 soldiers had died. He did not give any time period for those casualty counts.

The military claimed about 400 militants had been killed since Friday. Official casualty figures have not been independently confirmed from the offensive that began in late April.

Pakistani forces intensified attacks on the Taliban fighters in the Swat valley Monday, one day after thousands of residents fled the conflict zone during a temporary suspension in a curfew.

Officials said losses inflicted on the entrenched rebels Monday were not immediately available.

Around 90 per cent of residents of Mingora, Swat's main town, and nearly 35 per cent of the district's population have relocated, according to the English-language Dawn newspaper.

Aid agencies and charity organizations have warned of a worsening crisis, with at least one fearing "the largest man-made humanitarian disaster" since Pakistan's independence in 1947.

The United Nations refugee agency forecasts displacement of nearly 500,000 people, who are expected to join another 555,000 already uprooted because of the escalating violence since August.

According to Malik, authorities had registered 260,000 displaced persons from the Swat area through Sunday evening.

Civilians are being allowed to move out of the former tourism haven as urban warfare in the once-bustling streets seemed imminent.

But chances to flee come with the danger of being caught in crossfire or struck by roadside bombs planted by the insurgents. The military also accused the militants of hitting non-combatants with mortar fire.

Tens of thousands were leaving homes with only a few possessions they could carry, as the mass exodus caused a shortage of vehicles and compelled many evacuees to travel on foot.

Pakistan's forces have been pounding the Taliban fighters in towns and isolated mountain hideouts with artillery and aircraft to prepare the ground for head-on combat.

The intensifying offensive was expanded to Swat's adjoining district of Shangla at the weekend, where the army claimed killing at least 140 militants in an assault on a training camp.

Troops had also reported major gains in the neighbouring districts of Lower Dir and Buner.

The conflictive zones are part of the Malakand division in the North-West Frontier Province, which the regional government put under Islamic sharia law after signing a controversial peace agreement with the militants in February.

Defying earlier pledges to disarm, the militants moved closer to Islamabad, setting alarm bells ringing at home and abroad.

Separately, two paramilitary soldier and at least 10 civilians were killed Monday when a suicide car bomber attacked a security post near the provincial capital of Peshawar, police said.

The bomber rammed his vehicle into the checkpoint in Darra Adam Khel, a lawless town adjoining Pakistan's tribal region which is considered a hotbed of al-Qaeda and Taliban militants. A bus carrying civilians was caught in the explosion.

Twenty-six people were wounded in the blast, 13 soldiers and an equal number of civilians, police said. (dpa)