KOT ADDU, Pakistan — Floodwaters surged into Pakistan’s heartland and swallowed dozens of villages today, adding to a week of destruction that has already ravaged the mountainous northwest and killed 1,500 people.

The rush of muddy water over river banks in Punjab threatened to destroy vast stretches of crops that make the province Pakistan’s breadbasket, prompting the U.N. to warn that an estimated 1.8 million people will need to be fed in the coming weeks.

Adding to the misery, fresh rains in the northwest threatened to overwhelm a major dam and unleash a new deluge, while rescue workers struggled to deliver aid to some 3.2 million people affected by the floods despite washed-out bridges and roads and downed communication lines.

The government has struggled to cope with the scale of the disaster at a time when it is grappling with a faltering economy and a brutal war against the Taliban.

Several foreign countries and aid organizations have stepped in to support the government, including the United States, which announced today that it was sending six large military helicopters from Afghanistan to help with the relief effort.

But many flood victims have complained that aid is not reaching them fast enough or at all. That anger could grow as floodwaters surge through Punjab, Pakistan’s most populous province.

“We just ran away with our children, leaving behind everything,” said Fateh Mohammad, who was caught by surprise when water breached a protection bank in the Kot Addu area.

“All our possessions are drowned in the water. We have nothing,” said Mohammad, who was evacuated along with some 4,500 others by the army on boats and helicopters.

Water levels were so high in large tracts of Kot Addu and the nearby area of Layyah in the south of the province, that only treetops and uppermost floors of some buildings were visible, footage shot by an Associated Press Television News cameraman on a helicopter showed. People sought refuge on rooftops and tried to bring their livestock up as far as possible.

Ghulam Mustafa, a resident of a small town near Kot Addu, said he was also surprised by the flooding and had to leave behind three of his children who live at a religious school when he escaped with the rest of his family.

“It was a massive flood when we woke up early in the morning,” said Mustafa. “The army says they have rescued everybody in my village, but I do not know where my children are.”

Some residents piled their belongings on makeshift rafts built out of their furniture, which they swam through the huge tracts of muddy water.

Punjab is home to many of Pakistan’s largest farms, and the loss of so many crops was one reason the U.N.’s World Food Program estimated 1.8 million Pakistanis would need food assistance for at least the next month.

Aside from farmers, many victims have small businesses that have been destroyed and won’t have the means to buy food for their families, WFP spokeswoman Emilia Casella said in Geneva. The agency has delivered food to 40,000 people and is aiming to reach 250,000 by the end of the week, but “access is really remaining a major challenge,” she said.

In the northwest, new downpours threatened to exacerbate flooding that was already the worst in generations. Of the 3.2 million people affected by flooding, 2.5 million live in the northwest, UNICEF spokesman Marco Jimenez told reporters in Geneva.

Rising water levels at Warsak Dam, the country’s third biggest, prompted disaster officials to ask residents in the northern outskirts of Peshawar city to leave their homes.

“If needed, forced evacuation will be started,” said Adnan Khan, a spokesman for the Disaster Management Authority of Khyber Pakhtunkhwa province.

Rain is forecast for the next few days in the province and also in Punjab, said the head of Pakistan’s meteorological department, Qamar-us-Zaman Chaudhry.

Many people in the northwest were caught by surprise when extremely heavy monsoon rains first triggered floods a week ago, including Bibi Shehnaz, who lost her young son as she tried to escape rising water in the town of Charsada.

“While we were sleeping we heard loud voices of villagers warning to run because floods were coming,” said Shehnaz. “While I was trying to flee, my child slipped from my hands and was swept away by the gushing water.”

Around 1,500 people have been killed so far in the flooding, said Christian Cardon, a spokesman for the International Committee of the Red Cross.

More than 100,000 people are at risk of disease and clean water is urgently needed in flooded areas, said Fadela Chaib, a spokeswoman for the World Health Organization in Geneva.

Both the U.S. and the U.N. have pledged $10 million to help Pakistan deal with the disaster and have provided relief supplies.

The U.S., which is keen to win friends in the country, has also helped Pakistan survey flood-stricken areas with reconnaissance aircraft and plans to send six military helicopters from Afghanistan to help with the relief effort, the U.S. military said in a statement.

“It is vitally important we try to help those who have been tragically affected by the massive flooding,” said the top NATO commander in Afghanistan, Gen. David Petraeus.

But the U.S. has competition. At least one extremist group — a welfare organization allegedly linked to the Lashkar-e-Taiba militant network — is also helping survivors. The group, Falah-e-Insaniat, has previously helped civilians after other disasters.