U.S. officials said about 90 U.S. citizens remained unaccounted for Thursday, a day after catastrophic flash floods engulfed a border area of China and Nepal, killing hundreds of people.

In a statement, the U.S. State Department said it is not aware of any U.S. deaths, but three Americans have been rescued.

Many of the missing were on a pilgrimage to Mount Kailash in Tibet, a sacred site for Hindus, Buddhists and others.

Among those on a guided tour was Venus Rajnikant Patel, a 45-year-old Indian-born U.S. citizen who recently moved from St. Louis to Cincinnati to help his family.

Patel’s friend Jolene Gosha said she and her friends knew Patel was in Nepal and were devastated when they realized he was staying at a hotel along the border where the floods swept through.

“We’re just hoping, hoping that by some chance he was able to climb his way out of it,” Gosha said. &...