TOKYO — Amid rising tensions over the American military presence on Okinawa, the United States officially returned nearly 10,000 acres of land on the northern part of the island to Japan on Thursday. It was the largest transfer of land since the United States returned Okinawa to Japan in 1972, at the end of postwar occupation.

The United States ambassador to Japan, Caroline Kennedy, described the move as a step toward shrinking the American military presence on the main island of Okinawa. Nearly half of the roughly 50,000 American troops in Japan are stationed on the island, and the United States military controls about one-fifth of it.

At a ceremony in the seaside resort town of Nago on Thursday, Ms. Kennedy described the handover as a “milestone,” adding that “it also demonstrates America’s continued determination to reduce the impact of our presence here in Okinawa while maintaining our security commitments to the entire nation of Japan.”

Like many encounters between the United States military and residents of Okinawa, however, the land handover has stirred controversy. In exchange for the return of about half the acres that the American military uses to train soldiers for jungle warfare, the Japanese government agreed to build six helicopter landing pads on the land that the United States will retain. A small but fierce group of residents has complained about increased noise from construction and expressed concern about possible accidents.