A US Navy helicopter crashed on Thursday in Oman during a transport mission, and the fate of its five-member crew is unknown, the US military said, ruling out an attack.

The MH-53E Sea Dragon, used for long-range mine sweeping, crashed 58 miles (90 kilometers) southwest of the Omani capital Muscat “while conducting heavy lift support operations,” the Bahrain-based US Fifth Fleet said.

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The Fifth Fleet monitors the waters of the Gulf and keeps an eye on Iran.

“The crash was not due to any sort of hostile activity and the status of the five crew members is still being determined,” the statement said.

Another MH-53E was on the scene to assist the search and rescue operation.

The US Navy has boosted its mine-sweeping operations in recent weeks in response to Iran’s repeated threats to close the Strait of Hormuz, through which a fifth of the world’s traded oil passes.

It was not immediately clear whether the helicopter that crashed, assigned to Helicopter Mine Countermeasure Squadron 15, was part of the reinforcements.