Islamabad, Pakistan (CNN) -- The United Nations issued an urgent plea Wednesday for helicopters as the death toll from weeks of massive flooding in Pakistan reached 1,600 people.
"We need at least 40 additional heavy-lift helicopters, working at full capacity, to reach the huge numbers of increasingly desperate people with life-saving relief," Marcus Prior of the World Food Programme said in a statement.
The United States will send four more helicopters and an amphibious ready group to Pakistan, U.S. Army Brig. Gen. Michael Nagata said from Pakistan Wednesday.
By next week, at least 23 U.S. helicopters will be in the country ferrying relief supplies and rescuing people in and around the Swat River valley in northern Pakistan, Nagata said.
Floodwaters have washed away critical roads and bridges throughout the country, but especially in the mountainous areas of Gilgit Baltistan and Pakistani-administered Kashmir, and in the Swat Valley of Khyber Pakhtunkhwa province.
"In northern areas that are cut off, markets are short of vital supplies, and prices are rising sharply. People are in need of food staples to survive," Prior said.
The United Nations estimates that 800,000 people in need of humanitarian aid across Pakistan are accessible only by air.
The United Nations Children's Fund -- UNICEF -- and its partners estimated Wednesday that 2.5 million people in flood-devastated areas of the country have access to clean water. But an estimated 3.5 million have only contaminated water for drinking, bathing and washing, which puts them at risk for diarrheal diseases.
"Clean water is vital," said Martin Mogwanja, humanitarian coordinator in the South Asian nation, in a news release.
So far, 3.2 million people have received medical attention in affected areas since the floods began last month, the United Nations said. In all, 462,000 cases of diarrhea, 596,700 cases of skin diseases, and 441,000 of acute respiratory infections have been tallied, it said.
And nearly 65,000 cases of malaria have been reported in the south, especially Baluchistan and Sindh, where stagnant water provides breeding grounds for mosquitoes.
Over three weeks, Pakistan's floods have claimed nearly 1,600 lives, the national disaster management authority said Wednesday. They have affected more than 17 million people, leaving some 4 million homeless. An estimated 6 million people are in need of emergency shelter, of which just over 1 million have received tents or plastic tarps.
And thousands of people are still fleeing.
In Shahdadkot, floodwaters were pouring out of breaches, threatening to drown the entire city of 400,000 people, the United Nations' refugee agency said. In Sindh's Thatta district, 150,000 displaced people are in need of shelter.
About 80 percent of the area around the Sindh town of Jacobabad was underwater and people in neighboring Balochistan province feared they will be hit equally hard. In Gandhaka, people climbed to rooftops, said a release from the Office of the U.N. High Commissioner for Refugees.
Adding to the misery, the forecast calls for more monsoon rain across upper Pakistan through Thursday.
The United Nations says it still needs $200 million more in aid to cope with the crisis.
CNN's Ed Payne and Larry Shaughnessy contributed to this report.