The United Nations made an urgent plea for helicopters as the death toll from weeks of massive flooding in Pakistan neared 1,600 people Wednesday.

“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, 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.

The United Nations estimates that 800,000 people in need of humanitarian aid across Pakistan are accessible only by air.

Over three weeks, Pakistan’s floods have claimed 1,589 lives, the national disaster 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.

(Read More: CNN)