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

Tens of thousands of people remain in the region where the army is carrying out its campaign against the Taliban. A peace deal fell apart earlier in the year.

'Human suffering'

More than two million people fled the military offensive, but those left behind are unable to leave because of the fighting and because the military has surrounded towns and blocked off valleys.

In depth

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 Q&A: The struggle for Swat
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 The fight for northwest Pakistan
Talking to the Taliban
Pakistan's war
 Witness: Pakistan in crisis

The massive displacement caused poses not only a major burden on the economy, being kept afloat by a $7.6bn International Monetary Fund loan, but could undercut public support for the offensive.

Adams said: "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.

"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."

There was no immediate comment from the military.

Battle for Mingora

Last week, Lieutenant-General Nadeem Ahmed, who heads the government's relief operation, said that up to 200,000 civilians are stranded and that the authorities might have to drop food to them.

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

In Swat's main city of Mingora, soldiers are moving from house to house battling fighters. Clashes are also taking place in other parts of the valley, according to military reports.

The military has said its operation in Mingora, where between 10,000 to 20,000 civilians are thought to still be trapped, will be a slow process "to avoid civilian casualties".

Further attacks

A Taliban commander reportedly ordered his fighters to leave the city on Monday, saying the move was necessary to prevent civilian casualties.

Major-General Athar Abbas, a military spokesman, dismissed the Taliban's call as a  "ploy" to allow their fighters to escape.

Elsewhere in Pakistan's North West Frontier Province on Tuesday, two blasts killed a man and wounded five people.

One roadside bomb targeted a police van patrolling in Tank district, near the tribal area of South Waziristan, injuring two policemen.

In a separate attack in the nearby region of Dera Ismail Khan, a hand grenade was lobbed into the home of a Shia family, killing one man and wounding three others, a police official in the region said.