Washington (CNN) Iran has begun a major military exercise in the Persian Gulf region according to a US defense official directly familiar with the latest US information.

The "exercise is underway," and the US assesses the Islamic Revolutionary Guard Corps has deployed "dozens" of small boats in the initial phase. The exercise is taking place in the Persian Gulf, the Strait of Hormuz and the Gulf of Oman.

While Iran has not widely publicized the exercise, the US military is concerned it could demonstrate Iran's ability to interfere with commercial shipping and shut down the Strait of Hormuz, a crucial conduit for global energy supplies, US officials say.

The timing is unusual. These types of IRGC exercises typically happen much later in the year and the official said the US assesses that the exercise is partly being held in response to rising rhetoric from the US.

The official said it's not clear if the timing of the exercise is tied to the US only having one warship inside the Gulf at this time. US military commanders are "monitoring the situation closely" a second official said. Additional US Navy ships are expected to move the Strait of Hormuz and into the Gulf in the coming days officials say.

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