Islamabad, Pakistan (CNN) -- A second wave of floodwater is expected to wash through southern Pakistan this weekend, adding hundreds of thousands of people to the number already stranded by a first wave earlier this month, Pakistani officials said.
At least 1,384 people have died and another 1,630 have been injured by flooding that has damaged or destroyed 8,300 Pakistani villages so far, according to National Disaster Management Authority spokesman Ahmed Kamal.
The United Nations said it was racing against time to reach the suffering.
"Relief supplies must reach women, men and children as soon as possible, in order to avoid further death caused by waterborne diseases and food shortages," said Martin Mogwanja, the United Nations' humanitarian coordinator for Pakistan.
"The death toll has so far been relatively low compared to other major natural disasters, and we want to keep it that way," he said.
Two of 19 military helicopters pledged by the United States arrived in Islamabad, Pakistan, on Thursday to help in the humanitarian relief efforts, a U.S. Embassy spokesman said.
These aircraft replace six U.S. Army helicopters that will return to duty in Afghanistan, the embassy spokesman said. The Army helicopters rescued more than 3,000 people and delivered relief supplies since arriving in Pakistan on August 5, the spokesman said.
Farther north, more than 100 U.S. troops are on the ground in the volatile Swat valley, helping the Pakistani government deliver supplies and rescue people from flood-ravaged areas. None have yet encountered Taliban fighters.
"Obviously there's a militant threat in this region, but the Pakistani military has done an incredibly committed job -- providing us multiple layers of security around our units," said U.S. Army Brig. Gen. Michael Nagata in a conference call from Pakistan. "All our activities and helicopters are completely safe."
The bigger problem, said Nagata, is the weather. In the last two weeks, air crews have only been able to fly five days because of the relentless monsoon rains.
The initial wave of the flooding came after torrential rains two weeks ago filled rivers in northern Pakistan that swept southward. Weather experts said a year's worth of rain fell in a day at the peak of the relentless downpours.
A second wave of floodwater from rain that fell in northern and central Pakistan the next week is now reaching the Sindh province in southern Pakistan, according to CNN meteorologist Brandon Miller.
An apparent bottleneck of water from the first flood wave at the Sukkar dam could be overtaken by the second wave, Miller said.
"This could lead to a situation where the two flood waves could build upon each other and lead to even more catastrophic flooding," Miller said.
Pakistan emergency officials predicted the second wave could reach Sukkar on Saturday evening.
Hundreds of thousands of people living along the Indus River in northern Sindh could be stranded along with the tens of thousands stranded by the first wave, said Pakistani Navy Lt. Commander Jawad Khawaja.
Many residents have ignored government warnings to evacuate the area, causing a big concern, Khawaja said.
More than 14 million Pakistanis are affected, with 400,000 rescued from floodwaters, the government said. It estimated that 723,950 homes have been damaged or destroyed.
In the mountainous north, water funneled down treacherous slopes, quickly engulfing the roads below. It surged down the Indus River, spreading economic, political and social woes through the heart of Pakistan.
The smell of bodies filled the air in many places, where mere seconds of unstoppable water ended lives. Many witnesses, even those who thought they had seen everything in this land marred by crisis and killing, said they were stunned by the suffering wrought by the monsoons.
U.S. President Barack Obama, in a statement marking the 63rd anniversary of Pakistan's independence on Friday, said, "We will remain committed to helping Pakistan and will work side by side with you and the international community toward a recovery that brings back the dynamic vitality of your nation."
CNN's Chris Lawrence and Samson Desta contributed to this report.