(CNN) Five Iranian boats fired shots across the bow of a Singapore flagged cargo vessel in the Persian Gulf on Thursday in an attempt to potentially stop the ship, a U.S. official told CNN. For the first time, the incident brought another Persian Gulf nation into the recent rising maritime tensions in the region.

It is not yet clear if any of the rounds hit the Alpine Eternity. There were no U.S. citizens or cargo on board. The Pentagon is still gathering information about the incident.

The incident began when five small fast boats, believed to be manned by Iran's Revolutionary Guard Corps Navy approached the cargo vessel just off the coast of the United Arab Emirates but in international water, the official said.

These Iranian boats are typically manned with smaller caliber weapons such as machine guns.

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