The security details would provide a further layer of protection for the tankers in addition to the warships and planes operating in the strategically vital area, through which around a fifth of the world's oil passes.

"There is an effort... to employ security details composed of both Marines and Navy sailors on commercial tankers transiting in and near the Strait of Hormuz as an added layer of defense for these vulnerable ships," the official told AFP on condition of anonymity.

An invitation is needed to do so because the ships are private vessels, but "we are undergoing preparations to execute should final agreements be in place to do that," the official said.

The United States is boosting its forces operating in the region, announcing last month that it would deploy a destroyer, F-35 and F-16 warplanes, as well as an Amphibious Readiness Group/Marine Expeditionary Unit comprised of some 3,000 personnel.

The US military says Iran has either seized or attempted to take control of nearly 20 internationally flagged ships in the region over the past two years.

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 have 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.