(DARWIN, Australia) — At least three U.S. military personnel were killed on Sunday when a Marine helicopter crashed during a training exercise off the northern coast of Australia, U.S. officials said.

The Bell Boeing V-22 Osprey was carrying 23 service members when it went down at about 9:30 a.m. near the Tiwi Islands, in the Northern Territory of Australia, officials with the Marine Rotational Force-Darwin said in a statement.

Five other service members were taken to the Royal Darwin Hospital in serious condition, the statement said.

The Osprey had been transporting troops as part of a “routine training exercise” in support of Exercise Predators Run, a joint military exercise, officials said.

Recovery efforts were ongoing, officials said. The cause of the crash had not yet been determined.

U.S. Army officials in April had ordered a 24-hour stand-down of all aviation units following two deadly mid-air helicopter collisions within 30 days.

“The safety of our aviators is our top priority, and this stand-down is an important step to make certain we are doing everything possible to prevent accidents and protect our personnel,” Army Chief of Staff Gen. James McConville said in a statement announcing the stand-down.

ABC News’ Edward Szekeres, Victoria Arancio and Luis Martinez contributed to this report.

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