REUTERS - The United States is increasingly concerned about training by Iran's elite Revolutionary Guards for the Houthi rebels in Yemen, where the Shi'ite militias continue to make territorial gains despite airstrikes by neighboring Saudi Arabia.

U.S. officials said Tehran's direct involvement with the Houthis was limited but that U.S. intelligence assessments had concluded that Islamic Revolutionary Guard Corps personnel were training and equipping Houthi units.

The officials, who spoke on condition of anonymity to discuss security matters, expressed concern that the IRGC's mission could include training the Houthis to use advanced weaponry they acquired after seizing Yemeni military bases.

Saudi Arabia, which launched aerial bombardments of Houthi forces this week, has said the militia was receiving extensive backing from Iran, the kingdom's regional rival.

"We see ... Iran playing a large role in supporting the Houthis," Saudi ambassador to Washington Adel al-Jubeir told reporters on Thursday.

"There are Iranian advisers advising them and Hezbollah operatives advising them," Jubeir said. Lebanon-based Hezbollah is closely allied with Tehran.

Asked about Jubeir's accusations on Friday, White House spokesman Josh Earnest told reporters: "We've expressed our previous concerns about the destabilizing impact that Iran is having on this particular situation. We continue to have those concerns."

The Houthis, whose home territory is in northern Yemen, practice Shi'ite Islam, the majority faith in Iran.

In September, they seized Yemen's capital, Sanaa, pushing aside President Abd-Rabbu Mansour Hadi. Last month, Hadi fled to the southern port city of Aden and on Friday traveled to Egypt for a summit of the Arab League in Sharm el-Sheikh.

U.S. officials have long debated the extent of Iran's support for the Houthis. Some say the aid is largely opportunistic and not a top priority for Tehran at a time it is also backing Shi'ite militias in Iraq and supporting embattled Syrian President Bashar Assad.

A Houthi official told Reuters on Thursday that the group was prepared to confront the Saudi-led airstrikes without calling on Iran's help.

"The Yemeni people are prepared to face this aggression without any foreign interference," said Mohammed al-Bukhaiti, a Houthi politburo official.

Riyadh's intervention is aimed at blunting perceived Iranian influence.

The Saudis "cannot accept the idea of an Iranian-backed regime in control of Yemen, which is why they felt compelled to intervene the way they have," British Foreign Secretary Philip Hammond said on Friday during a visit to Washington.

Jubeir, in his remarks to reporters, noted that the Houthis had seized sophisticated weapons, including ballistic missiles.

"It's no secret," he said, that the Iranians "are providing assistance and support to the Houthis, both political as well as militarily, as well as economic."

"The first thing the Houthis did when they entered and occupied Sanaa was to free Iranian Revolutionary Guards operatives and Hezbollah operatives from the jails," he said.