U.S. officials claim that Iran’s Islamic Revolutionary Guard forces are laying the groundwork to begin a major exercise in the Persian Gulf, a move that could show off their capability of closing the Strait of Hormuz, a new report says.

The Strait of Hormuz connects the Persian Gulf and the Arabian Sea, and is considered paramount to international shipping. The U.S. has urged surrounding nations such as Saudi Arabia to remain firm in keeping the waterway open amid threats from Iran to close it.