A U.S. military helicopter crashed on Monday inside an American base on the southern Japanese island of Okinawa, Japanese officials said, injuring at least one of the crew and sending up a smoke cloud that could be seen for miles.

The accident provides new fodder to Okinawan politicians and activists who say their island, which hosts half of the 50,000 U.S. troops stationed in Japan, is endangered by U.S. exercises and aircraft that frequently pass over tightly packed neighborhoods.

A U.S. Air Force statement said an HH-60 helicopter in the middle of a “local training mission” crashed inside Camp Hansen, in the center of the island.

Japan’s defense minister, Itsunori Onodera, speaking to reporters in Tokyo, said three crew members were confirmed safe and one was taken to the hospital, according to the Associated Press.

Onodera called the incident “regrettable” and asked for an investigation to prevent a recurrence.

Discontent in Okinawa about the presence of U.S. troops is one of the challenges Washington faces as it tries to realign military bases in the region.

Monday’s accident brought to mind a 2004 crash in which a U.S. helicopter based at Marine Corps Air Station Futenma fell from the sky onto the grounds of Okinawa International University. The helicopter clipped an administrative building.

The Marine base is located adjacent to the university campus in the middle of the densely populated city of Ginowan. For two decades, U.S. and Japanese officials have tried to relocate it.

They agree that crowded Ginowan is no place for combat helicopters and massive transport planes. But the relocation of the base has been blocked by local opposition to the construction of a replacement facility in a less populated northern strip. More recently, the Senate Armed Services Committee also raised concerns about the relocation plan, citing the cost.

Since 1972, U.S. aircraft have been involved in 44 crashes on Okinawa, said NHK, Japan’s public broadcaster. Protests erupted in Okinawa last year when the United States deployed a new fleet of MV-22 Ospreys — aircraft that can take off like helicopters and fly like planes.

Japanese defense officials in Tokyo were in favor of the Ospreys, calling them an effective means to help defend islands that are claimed by both Japan and China. But Okinawans worry about the Osprey’s safety record, which includes several crashes.