Pakistan will need billions of dollars to recover from massive flooding that has affected an estimated 13 million people and killed 1,500, the UN said Sunday.

Jean-Maurice Ripert, the UN special envoy for flood relief in the country, said foreign aid is desperately needed for both immediate food, shelter and medical-care needs, as well as long-term reconstruction.

While the agency cannot yet say exactly how much money will be needed, Ripert told The Associated Press that "the emergency phase will require hundreds of millions of dollars and the recovery and reconstruction part will require billions of dollars."

The UN's plea to the international community came on a day when a government official announced that landslides caused by the heavy monsoon rains killed at least 53 people on Saturday in the Gilgit-Baltistan area.

Ali Mohamamd Sikander said Sunday the landslides hit the villages of Kumra, where 37 people died, and Ghanche, where 16 residents were killed.

Meanwhile, rescue crews raced Sunday to evacuate thousands of residents in southern Sindh province, which was submerged as floodwaters gushed down from the northwestern mountains.

The worst of the devastation is in the northwest province of Khyber Pakhtunkhwa, which sustained heavy damage from flooding caused by the heavy and relentless monsoon rains. The floodwaters have since poured south into central and southern provinces such as Punjab and Sindh.

So far, the U.S. government has pledged millions of dollars and provided six military helicopters to both evacuate victims and distribute badly needed food and water. However, ongoing rains have grounded aid flights, said NBC News' Stephanie Gosk.

"Of the most concern right now for the government here and for the military are the some 100,000 people who are stranded in the mountainous areas of northwest Pakistan who were completely cut off when bridges were swept away over a week ago," Gosk told CTV News Channel Sunday morning in a telephone interview from Pakistan.

"It's been difficult for them to get there because across the country there have been heavy downpours, the weather is not good and they can't make the helicopter flights that are necessary to bring aid to those people."

Gosk said a massive humanitarian crisis is brewing in the region, as residents left homeless by the floodwaters are living in sub-standard tents with little food or water and disease is spreading rapidly.

The flooding has also impacted Pakistanis not directly affected by the rains and floodwaters in the form of skyrocketing food prices.

The cost of fruit and vegetables in particular has soared after flooding wiped out vast swaths of crops. The prices of staples such as tomatoes, onions, potatoes and squash have as much as quadrupled in recent days, making them too expensive for many Pakistanis.

"It is like a fire erupted in the market," Mohammad Siddiq said at a vegetable stand in Lahore. "Floods and rains have made these things unaffordable."

About 570,000 hectares of crops were destroyed in Punjab alone, the region considered Pakistan's breadbasket. More crops were destroyed in the northwest.

"The flooding has caused massive damage to crops and also to the reserve that people had at their houses," said Amjad Jamal, a spokesperson for the World Food Program.

"Khyber Pakhtunkhwa was a food insecure province even before the floods, and a lot of areas are such that people can't afford even one meal a day."

With files from The Associated Press