Pakistani forces attacked Taliban fighters in the Swat valley with artillery and helicopters today (May 6) after the US called on the government to show its commitment to fighting militancy.

Expanding Taliban influence in nuclear-armed Pakistan has spread alarm at home and abroad and will be a core issue when US President Barack Obama meets his Afghani and Pakistani counterparts in Washington later on Wednesday.

A February peace pact aimed at ending Taliban violence in Swat is in tatters and thousands of people fled from Mingora, the region's main town, on Tuesday after a government official said fighting was expected.

The militants have captured several important government buildings in the town, 130 km (80 miles) northwest of Islamabad, and took positions on rooftops.

While a curfew kept people off the streets, government forces attacked in and around the town, including at an emerald mine the Taliban have taken over.

'Security forces have engaged militants' positions at an emerald mine and helicopter gunships are also being used to flush militants out of Mingora,' said a military spokesman.

A military official who declined to be named brushed off speculation the clashes signaled an imminent major offensive in Swat but residents said they saw troops being trucked in and a government official also said reinforcements were arriving.

Authorities estimate 500,000 people could flee from the valley, provincial information minister Mian Iftikhar Hussain said on Tuesday.

A similar number has already fled fighting in different parts of the northwest since August, putting an extra burden on an economy propped up by a $7.6 billion International Monetary Fund loan.

'Running out of food'

President Asif Ali Zardari, former prime minister Benazir Bhutto's widower, is due to meet Obama and Afghan President Hamid Karzai for talks on the growing militant threat in the region.

Increasing violence and the spread of the Taliban have raised doubts about the ability of the civilian government elected last year to face up to the militants.

'Pakistan must demonstrate its commitment to rooting out al Qaeda and the violent extremists within its borders,' Richard Holbrooke, US special representative for Afghanistan and Pakistan, said in congressional testimony on Tuesday.

Pakistani action against militant enclaves on the Afghan border is vital to efforts to stabilise Afghanistan.

Some residents of Mingora said they faced dwindling supplies of food and were desperate to get out.

'We are very scared. We want to go as soon as possible but can't because of curfew,' said grocery shop worker Gul Nazir.

'We're running out of food. We don't know what to do.'     Pakistani stocks have been hurt in recent sessions by worries about security but the main index was nearly 2 percent higher in thin early trade.

The Swat peace pact, under which authorities agreed to a Taliban demand for introduction of Islamic sharia law in the former tourist valley, led to accusations from critics the government was caving in to militant aggression.

The Taliban refused to give up their guns and last month pushed into Buner district, only 100 km (60 miles) northwest of Islamabad, and another district adjacent to Swat.

With worry intensifying, security forces launched an offensive to expel militants from Buner and another district on April 26. About 180 militants have been killed, according to the military, although there has been no independent confirmation.

At the Washington talks, Obama will present Zardari and Karzai with his strategy for defeating al Qaeda. Zardari will stress his government is on the right track and needs help.

Critics says Pakistan has been in denial about the Taliban threat in a country where old rival India ha