Commander Bill Urban, spokesman for the Navy's Bahrain-based Fifth Fleet, said there had been no "unsafe or unprofessional" interactions with the Iranians at sea since August 14, 2017 when an Iranian drone with no lights on flew close to US aircraft operating in the Gulf.

It "is a substantial period time since then, and something that we think is great," Urban told reporters.

Last year and in 2016, the US Navy complained repeatedly about the behaviour of Iranian Revolutionary Guard vessels, which would often shadow and steer towards US ships.

In at least one incident, US sailors had to fire flares and warning shots before the Iranians turned away.

Urban said that since then, the Iranians have stopped approaching so closely.

"We have seen an across-the-board change in behaviour," Urban said.

"I don't necessarily have a reason for that but it's pretty clear that it's something they are consciously doing."

The change comes amid increased rhetoric from Washington about Iran's "malign influence" in the region and US President Donald Trump's persistent railing against the 2015 Iran nuclear deal.

The Fifth Fleet and its associated task forces patrol the Gulf continuously and inspect some of the ships passing through.

In 2016, navy personnel seized weapons, including machine guns and rocket launchers, they suspected were headed from Iran to Yemen.

Urban said task forces this year have seized record amounts of heroin, some of which may have been from the Taliban, Afghanistan's biggest militant group.

The Iranian Revolutionary Guards is a paramilitary force that answers directly to the Islamic republic's supreme leader Ayatollah Ali Khamenei.

In January 2016, the Iranians briefly captured the crew of two small US patrol boats that strayed into Iranian waters.

The 10 US sailors were released 24 hours later.