By Phil Stewart and Yara Bayoumy
WASHINGTON (Reuters) - When U.S. Defence Secretary Jim Mattis greeted Saudi Arabia's foreign minister at the Pentagon last month, the first thing he did was joke about the time "the Iranians tried to murder you."
Mattis' reference to a foiled 2011 plot, denied by Iran, was a telling sign of how much more aligned President Donald Trump's administration is with Gulf allies about what they perceive to be the Iranian threat, a shift that seems to be setting the stage for greater U.S. involvement in Yemen, in particular.
After long seeking to distance itself from Yemen's brutal civil war, the United States under Trump now appears increasingly to see the conflict through the Gulf's prism of Iranian meddling, even as Washington prioritises a parallel fight against al Qaeda.
Detailed discussions are under way within the Trump administration that would offer greater aid to Gulf allies fighting Iran-aligned Houthi rebels. Officials say that could included expanded sharing of U.S. intelligence.
In Saudi Arabia last week, Mattis compared Tehran's backing for the Houthis to its support for Shi'ite ally Lebanese Hezbollah, a view long espoused by Saudi Arabia and other Gulf states who see links between the two groups.
"Everywhere you look, if there is trouble in the region, you find Iran," Mattis told reporters in Riyadh.
Iran rejects Saudi accusations that it is giving financial and military support to the Houthis in the struggle for Yemen.
Cooperation between the United States and the Gulf is already on the rise in the fight against America's top priority there: al Qaeda in the Arabian Peninsula.
U.S. officials see that broader civil war as an obstacle to a sustained military campaign against the militants, as well as a threat to the Bab al-Mandab strait, a strategic waterway.
IRAN MUSCLES INTO YEMEN
For many U.S. and Gulf observers, the role of Iran is clear in the increasing sophistication of Houthi forces.
The United States launched cruise missiles at Houthi targets last year after a U.S. warship came under fire off Yemen, and a remote-controlled boat laden with explosives rammed a Saudi frigate on Jan. 30, the first known strike by a "drone" vessel.
"These weapons didn't exist ... before the war. There was no explosive boat that existed in the Yemeni inventory," Vice Admiral Kevin Donegan told Reuters in an interview.
Donegan, commander of U.S. Naval Forces Central Command, also said ballistic missiles fired into Saudi Arabia had several times the range of missiles the Yemenis had before the conflict.
"When you have a non-nation-state with nation-state-like weapons that can reach into the maritime (area), it has my attention," Donegan said.
A senior UAE official also compared the Houthis to Hezbollah, and said growing Iranian support had helped shield them from pressure to enter political talks.
"We are seeing UAVs, anti-tank and anti-ship missiles, as well as land and sea mines," the official told Reuters.
Mattis has publicly backed a political solution to the conflict. But U.S. officials have also acknowledged that Saudi-led coalition military pressure could help create the conditions for that negotiated solution.
Still, the Yemeni government and the Gulf-led coalition have been preparing for a possible assault on Hodeidah port, the entry point for nearly 80 percent of Yemen's food imports, because they say the Houthis use it to smuggle weapons and ammunition.
The Yemeni government has proposed to the United Nations that it monitor Hodeidah.
Trump administration officials have not ruled out U.S. assistance should the coalition move ahead, but U.S. officials said Washington is not considering strikes on Houthi targets or deploying ground forces.
Eric Pelofsky, who helped craft Yemen policy for Obama, warned an assault risks tipping Yemen into a terrible famine and has proposed that coalition forces perhaps instead strengthen their offensive near Kitaf to increase pressure on the Houthis.
The United States has carried out more air strikes this year in Yemen than in all of 2016, in a clear sign of growing U.S. concern and focus on AQAP and deepening involvement in Yemen.
Largely unseen, however, is the partnership with the United Arab Emirates that is helping guide that increased activity.
UAE-backed forces have captured al Qaeda operatives, and while a January raid in Yemen was Trump's first and so far only one since he took office, UAE-trained local Yemeni forces have carried out more than 250 raids in and around Aden, a senior coalition military commander told Reuters.
The port of Mukallah was similarly retaken last year with 10,000 troops, largely Yemenis trained by the coalition.
"We are encouraged by U.S. support and the change from a drone policy to greater engagement. We can't really defeat and dislodge al Qaeda with a drone policy," the senior UAE official said.
Intelligence gleaned from UAE-backed operations has been shared with the United States, which is rebuilding its knowledge about the group since the war forced the pullout of its personnel in 2015.
"In terms of the U.S. supporting us or the coalition supporting the U.S., it’s really mutual," the senior coalition military commander said, adding that the coalition has strong human intelligence, complemented by Washington's strong intelligence, surveillance and reconnaissance abilities.
Still, U.S. officials acknowledge there is a lot they don't know. Even the presumption about AQAP's size, seen in the low thousands, is a very low-confidence estimate.
Beyond things like increasing intelligence sharing, U.S. officials are moving closer to approving a sale of precision-guided munitions to Saudi Arabia blocked under Obama.
A recent meeting between Trump and Saudi Arabia's deputy crown prince Mohammed bin Salman was dubbed a turning point in relations which had been frosty under Barack Obama, but some cautioned against too cozy a relationship.
Anne Patterson, former deputy assistant secretary of state for near east affairs under Obama, and who advocated selling the munitions, said the United States should also help Riyadh improve its targeting.
But some have expressed caution.
"It makes the U.S. complicit again in Saudi actions that are causing great suffering over which we have insufficient control," said Tom Malinowski, the state department's top human rights official during the Obama administration.
(Additional reporting by Idrees Ali, Editing by James Dalgleish)