US steps up intel and naval boarding to block Iran from providing missiles that would let Houthis foil Saudi-led airstrikes.

US officials revealed that Iran is trying to smuggle surface-to-air missiles into the Shi'ite Houthi militia in Yemen, to aid its attempts to conquer the country and stave off a Saudi-led airstrike campaign.

One of the senior US defense officials quoted in the Wall Street Journal on Sunday said of Iran's efforts to supply surface-to-air missiles: "we are looking. We know they are trying to do it."

The acquisition of such weapons by the Houthis is seen as a major concern, given that it would provide them leverage in challenging the Saudi-led airstrikes and allow them to more easily spread their control.

"They (Iran - ed.) don’t have an easy route in from the air. They don’t have an easy route in from the sea," one senior US military source told the paper. "There’s lots of intelligence focused on what they’re doing - from loading to potential delivery."

Those efforts to track Iran and prevent it from arming the Houthis have been stepped up by the US in recent weeks, with American naval forces in the Red Sea performing their first boarding operation this month as part of that campaign.

The USS Sterett destroyer searched the Panamanian-flagged Saisaban on April 1 and did not find any covert weapons, but the incident shows the American resolve not to let game-changing weapons enter Yemen.

There is an assessment that the Iranians will be limited in their involvement in Yemen at the current juncture in time, given that they want to seal a nuclear deal by June 30 that would ease sanctions.

"I don’t get the sense that the Iranian level of commitment at this point is of such a magnitude that they are going to take a big risk of being exposed any more than they already are,” a military official said. “If they can do it, and it’s not going to cost them a whole lot, I think they’ll do it. But the Houthis are not some ally that they are going to go to the mat for.”

While Iran has long denied arming the Houthis, evidence has shown their covert activities in Yemen have been ongoing for years.

Two years ago, Yemeni authorities seized a ship named the Jihan I off the coast - it was carrying a massive cache of weapons, including surface-to-air missiles and Katyusha rockets. The arms were thought to be from Iran and earmarked for the Houthis.

The Saudi-led airstrikes, which began roughly three weeks ago, are said to be slowing the Houthi advance on the key southern port city of Aden, but according to US officials they have yet to inflict a crippling blow on the Houthis.