Speaking to reporters as he returned from a trip to Oman, Afghanistan and Bahrain, Mattis said officials he met with had expressed frequent concerns about Iranian behavior.

"One thing that came through loud and clear is the suspicion of Iran and the evidence of Iranian destabilizing efforts," said Mattis, a longtime Iran hawk.

"I heard it when I was up in Afghanistan. You know what's going on in terms of Iran's support to Assad. Now Iran is following Russia's example (and) mucking around in Iraq's elections," Mattis said, referring to Syrian President Bashar al-Assad.

"It was just brought home to me again that they are not changing their behavior, they are continuing to be a destabilizing influence," Mattis added.

The Pentagon chief said he would not speculate as to whether Iran's efforts were having any impact on the Iraqi electorate ahead of the May parliamentary and provincial assembly elections.

"Iran is trying to influence using money the Iraqi elections. That money is being used to sway candidates, to sway votes," he said.

"Iran should leave the Iraqis to determining their own future," said Mattis.

Despite increased rhetoric from Washington about Iran's activities in the region and US President Donald Trump's continual railing against the Iran nuclear deal, Mattis noted that Iranian naval vessels in the Gulf have become less provocative toward US ships.

He 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 vessels.

"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."

- 'Testing' off Yemen -

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.

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

He said there has been "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 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."

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 confiscated 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.