Mattis said ships from both the regular Iranian navy and the Iranian Revolutionary Guards Corps have curtailed the sorts of incidents that had become almost routine over the past few years, and are now staying away from American ships.

"In the Gulf itself, they are not coming in as close to our ships, the provocative actions in the Gulf seem to have relented somewhat," Mattis said.

"They are not doing as many bellicose confrontations and that sort of thing."

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

Earlier, 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.

"We have seen an across-the-board change in behavior."

Last year and in 2016, the US Navy frequently complained about the behavior of Iranian Revolutionary Guard Corps vessels, which would often shadow and steer toward American 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.

Mattis, a longtime Iran hawk, accused Tehran of "destabilizing" behaviour across the region and said that off the Yemen coast around the Bab-al-Mandab strait, the Islamic Republic is testing a number of offensive capabilities.

"It's where you find (Iran's) radars, their ballistic missiles, anti-ship cruise missiles," Mattis said.

"We've found their mines, their explosive boats all being tested, increased capability being demonstrated down there."

- US patrols -

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

In 2016, sailors seized weapons apparently headed from Iran to Yemen, including machine guns and rocket launchers.

Urban said task forces this year have seized record amounts of heroin, much of which may have been grown in Afghanistan to fund the Taliban.

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

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.