Pakistani authorities advised the people of Swat valley, in the north west, to evacuate the district's main town Mingora Tuesday, as Taliban extremists took effective control of the place.

A humanitarian crisis now looks likely as the provincial government said that 500,000 people are expected to flee Swat.

Locals reported that Taliban fighters in charge of the streets of Mingora, having fought gun-battles with the local army base through the night.

Up to now, Pakistan had pretended that a controversial peace accord with Taliban in Swat was still holding. But the evacuation indicated that the situation was no longer manageable and that an army offensive against the militants could begin within hours.

The news came as Pakistan president Asif Zardari is due to arrive in Washington Tuesday, for his first formal meeting with Barack Obama. Obama has already said that Pakistan, a key US ally, is in a "very grave" redicament.

A war in Swat is likely to be bloody and could spill out of the region, which lies just 100 miles from Islamabad.