Islamabad, Pakistan (CNN) -- Two U.S. Navy MH-53E Sea Dragon helicopters arrived Saturday to assist with humanitarian and rescue efforts in flood-ravaged Pakistan, which canceled celebrations of its 63rd birthday.
A statement from the U.S. State Department says the two aircraft are part of the contingent of 19 helicopters, ordered to Pakistan on Wednesday by U.S. Secretary of Defense Robert Gates.
Seven of the 19 craft are now in the country. One other MH-53E Sea Dragon helicopter and four U.S. Marine Corps CH-53E helicopters arrived earlier this week.
Twelve Marine CH-46 Sea Knight helicopters will arrive over the next few days.
Since August 5, U.S. military helicopters have rescued more than 3,500 people and transported more than 412,000 pounds of emergency relief supplies, according to the State Department.
One-fifth of Pakistan -- which is about the size of Florida -- has been flooded in relentless monsoon rains, the United Nations says. Nearly 1,400 people have died and 875,000 homes have either washed away or are damaged, according to Pakistan's Disaster Authority.
Millions more are still at peril as the bloated Indus River is cresting this weekend in parts of Sindh province. In some areas, the Indus has expanded from its usual width of one mile to 12 miles.
Homes, crops, trees, livestock, entire villages and towns have been transformed into vast lakes.
The worst floods since Pakistan's creation have disrupted the lives of about 20 million people, Prime Minister Yousaf Raza Gilani said Saturday.
Surrounded by a tragedy of epic proportions, Pakistanis canceled Saturday's celebrations of independence, hard won from the British in 1947. They might have otherwise attended parades, burst firecrackers and waved the green and white flag proudly.
Instead, President Asif Ali Zardari, under fire for a perceived lack of government response, toured flood-ravaged Khyber Pakhtunkhwa in the north, where the crisis began more than two weeks ago. He urged Pakistanis to remember the afflicted.
"The best way to celebrate the Independence Day this year is to reach out to the victims and help them to help themselves," he said, according to the Associated Press of Pakistan.
"I stand with you and the people of Pakistan stand with you as well, in your hour of trial," he told flood victims in Seraiki. "Do not lose hope as the entire nation stands with you."
Gilani said that even some religious events connected to the Muslim holy month of Ramadan were canceled. He said money that might have been spent on Iftar, the feast that breaks the daily fast between sunrise and sundown, should be used to alleviate suffering.
Meanwhile, the United Nations said Secretary General Ban Ki-moon intends to visit Pakistan Sunday and see the devastation firsthand. His trip comes as U.N. and other humanitarian agencies are racing against time to prevent more death and 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 with other major natural disasters, and we want to keep it that way," he said.
Pakistan emergency officials predicted the second wave of floodwaters gushing down the Indus River could reach the southern town of Sukkar by Saturday evening.
Hundreds of thousands of people living along the Indus could be stranded, along with the tens of thousands stranded by the first wave, said Lt. Cmdr. Jawad Khawaja of the Pakistani navy.
Many residents have ignored government warnings to evacuate the area, causing a big concern, Khawaja said.
"The time to act is now -- this is a disaster of unimaginable proportions," said Nilofer Bakhtiar, head of the Pakistan Red Crescent Society.
Thousands of flood victims huddled in sludgy camps or in jam-packed public buildings. Others slept under the stars next to the cows, sheep and goats they managed to rescue from rising waters.
But when they might be able to return to dry lands at home remained a big question. Pakistan's monsoon season is only half way over and more rain is on the way.
CNN's Samson Desta and Reza Sayah contributed to this report.