SUKKUR, Pakistan – Bad weather grounded helicopters carrying emergency supplies to Pakistan's flood-ravaged northwest Friday amid warnings the worst monsoon rains in decades would bring more destruction to a nation already reeling from Islamist violence.

U.S. military personnel waiting to fly Chinooks to the upper reaches of the hard-hit Swat Valley were frustrated by the storms, which dumped more rain on a region where many thousands are already living in tents or crammed into public buildings.

Over the last week, floods have spread from the northwest down Pakistan, killing around 1,500 people and affecting more than 3 million others.

Much of the destruction has come from the mighty Indus River, which in better times irrigates vast swaths of farmland.

Some 30,000 Pakistan soldiers are rebuilding bridges, delivering food and setting up relief camps in the northwest, which is the main battleground in the fight against al-Qaida and the Taliban.Foreign countries and the United Nations have donated millions of dollars.

Also helping out are Islamist charities, including at least one with links to a banned militant organization.

The government has come under criticism for not doing enough, especially since President Asif Ali Zardari chose to go ahead with a trip to Europe at the height of the crisis.

Amal Masud, a spokeswoman for the National Disaster Management Authority, said all helicopters currently stationed in the northwest were grounded because of poor weather.

Saleh Farooqi, head of the Provincial Disaster Management Authority in southern Sindh province, said authorities had evacuated about 200,000 people from areas where floodwaters could hit, but many more were still living in the danger zone.

"About 500,000 people living near the Indus River do not realize the gravity of the situation, and they do not know how fast the water is rushing to their areas," he said.

River embankments were being strengthened and people evacuated from low-lying villages in the province's Sukkur district, regional Irrigation Minister Jam Saifullah Dharejo said.

About 85 U.S. military service people are taking part in the relief activities.

On Thursday, six helicopters picked up about 800 stranded people and dropped off emergency aid.

The United States is unpopular in Pakistan, and Washington is hoping the relief missions will help improve its image. But the missions could draw criticism from Islamist politicians and others in Pakistan who are hostile to the idea of American boots on the ground, even if they are helping after a disaster.

The U.S. military carried out larger operations in the aftermath of the 2005 Kashmir earthquake, as it did in predominantly Muslim Indonesia after the 2004 tsunami. Those missions were credited with boosting Washington's reputation there.