A boat of Iran's Islamic Revolutionary Guard Corps Navy operates in the Strait of Hormuz, on Monday. AP

The incident on Monday involving the Guard and the Navy comes as tensions remain high over stalled negotiations over Iran's tattered nuclear deal with world powers and as Tehran enriches uranium closer than ever to weapons-grade levels under decreasing international oversight.

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Meanwhile, the United Nations' nuclear watchdog said Iran now plans to enrich uranium through a second set of advanced centrifuges at its underground Fordo facility amid the standoff.

A boat of Iran’s Navy operates in close proximity to patrol coastal ship USS Sirocco on Monday. AP

The Cyclone-class patrol ship USS Sirocco and Spearhead-class expeditionary fast transport USNS Choctaw County found themselves in the close encounter with three Iranian fast boats while coming through the Strait of Hormuz to enter the Persian Gulf, the Navy said.

In a video released by the Navy's Bahrain-based 5th Fleet, a high-speed Guard Boghammar is seen turning head-on toward the Sirocco. The Sirocco repeatedly blows its horn at the Boghammar, which turns away as it closes in. The flare shot can be heard, but not seen, as the Boghammar passes the Sirocco with the Iranian flag flying above it.

The Navy said the Boghammar came within 50 yards (45 metres) of the Sirocco, raising the risk of the vessels running into each other. The overall encounter lasted about an hour, the Navy said.

The Guard's "actions did not meet international standards of professional or safe maritime behavior, increasing the risk of miscalculation and collision," the Navy said.

Iran did not immediately acknowledge the incident in the strategic waterway — a fifth of all traded oil passes through the strait.

Associated Press