The move came after the Iranian Navy tried to seize two merchant vessels in the Strait of Hormuz and the Gulf of Oman earlier this month, in one incident opening fire at one of the merchant vessels.

"In light of this continued threat and in coordination with our partners and allies, the (Defense) Department is increasing our presence and ability to monitor the Strait and surrounding waters," Deputy Pentagon Press Secretary Sabrina Singh said, calling on Tehran to "immediately cease these destabilizing actions that threaten the free flow of commerce through this strategic waterway."

On Friday, a senior defense official said the United States was flying A-10 Warthog warplanes over the Gulf, armed "with the kinds of munitions that would be useful to go against fast boats and moving targets."

As Iranian "fast attack boats, fast attack craft have increased as a maritime threat, we've tried a number of different ways to tackle them. And the A-10 has proven capable of that," the official added.

Washington said its forces blocked two attempts by the Iran to seize commercial tankers in international waters off Oman on July 5, while Tehran took control of a commercial ship the following day.

In April and early May, Iran seized two tankers within a week in regional waters, and Tehran was also accused of launching a drone attack against an Israeli-owned tanker in November 2022.

There has been a series of such incidents since 2018, when then US president Donald Trump pulled out of the Iran nuclear deal and reimposed crippling sanctions on the Islamic republic, sending tensions soaring.

Talks aimed at reviving the accord remain stalled, but have recently resumed with Oman as an intermediary.