ISLAMABAD - Hundreds of thousands of Pakistanis trapped by an offensive against the Taliban in Swat face catastrophe and authorities should lift a curfew to enable them to get out and for help to get in, a rights group said.

The offensive in the Taliban bastion of Swat, about 120 km (80 miles) northwest of Islamabad, has sparked an exodus of 2.3 million people, according to government figures, but about 200,000 people are believed to be still in the valley.

Severe shortages of food, water and medicine were creating a major humanitarian crisis for the trapped civilians, the U.S.-based group Human Rights Watch said.

People trapped in the Swat conflict zone face a humanitarian catastrophe unless the Pakistani military immediately lifts a curfew that has been in place continuously for the last week, said Brad Adams, the group's Asia director.

The government cannot allow the local population to remain trapped without food, clean water, and medicine as a tactic to defeat the Taliban, Adams said in a statement on Tuesday.

The army launched its most concerted effort against the Taliban after the militants, emboldened by a controversial peace deal, pushed out of the former tourist valley into neighbouring districts, including one just 100 km (60 miles) from Islamabad.

But the flight of so many civilians poses not only a major burden for an economy being kept afloat by a $7.6 billion International Monetary Fund loan, but could also undercut public backing for the offensive.

Soldiers are slowly moving from house to house battling militants in Swat's main town of Mingora, while clashes are also taking place in several other parts of the valley, the military says.

HUMANITARIAN CORRIDOR

Human Rights Watch said it was getting persistent reports of civilian casualties from army shelling and aerial bombardments as well as reports of summary executions of civilians by the Taliban.

The Pakistani government should take all possible measures including air drops of food, water, and medicine to quickly alleviate large-scale human suffering in Swat, Adams said.

Both sides should allow a humanitarian corridor that would let civilians escape the fighting and for impartial humanitarian agencies to evacuate and aid civilians at risk, he said.

Military spokesmen were not immediately available for comment.

The head of the government's relief operation, Lieutenant General Nadeem Ahmed, said last week up to 200,000 civilians have been stranded by the fighting and authorities might have to drop food to them by air.

Ahmed said most people still in the valley were in its northern reaches, which had been relatively calm, and authorities wanted them to stay put, rather than risk traveling through the war zone in and around Mingora to the south.

A top U.N. official said on Monday the United Nations was considering asking the military to temporarily halt its offensive to enable aid to get to trapped civilians.

A humanitarian pause is a subject of discussion ... it is obviously something that we would not shy away from asking for, Manuel Bessler, head of the U.N. Office for the Coordination of Humanitarian Affairs in Pakistan, told AlertNet.

Pakistan