U.S. military officials are concerned that Iran's support for Houthi rebels in Yemen could spark a confrontation with Saudi Arabia and plunge the region into sectarian war. 

Iran is sending an armada of seven to nine ships — some with weapons — toward Yemen in a potential attempt to resupply the Shia Houthi rebels, according to two U.S. defense officials. 

Officials fear the move could lead to a showdown with the U.S. or other members of a Saudi-led coalition, which is enforcing a naval blockade of Yemen and is conducting its fourth week of airstrikes against the Houthis. 

Iran sent a destroyer and another vessel to waters near Yemen last week but said it was part of a routine counter-piracy mission. 

What's unusual about the new deployment, which set out this week, is that the Iranians are not trying to conceal it, officials said. Instead, they appear to be trying to "communicate it" to the U.S. and its allies in the Gulf. 

It is not clear what will happen as the convoy comes closer to Yemen. Saudi Arabia has deployed ships around Yemen to enforce the blockade, as has Egypt. An official said the ship convoy could try to land at a port in Aden, which the Houthis have taken over. 

Although the U.S. is assisting with the Saudi-led air campaign, it is not participating in the naval blockade of Yemen, said U.S. Central Command spokesman Col. Pat Ryder. 

However, the U.S. Navy is in the region and has already "consensually boarded" one Panamanian-flagged ship in the Red Sea on April 1 on the suspicion it was illegally carrying arms for the Houthis. 

None were found, but the move raised alarm bells in Washington over an increasingly active U.S. military role in the conflict. The Pentagon indicated this week that more boardings could occur. 

"We will continue to vigilantly defend freedom of navigation and to conduct consensual searches in an effort to ensure that drugs, human trafficking, weapons trafficking and other contraband are limited," Army Col. Steve Warren said on Monday. 

Officials fear a naval confrontation between Iran and Saudi Arabia could escalate what has become a proxy war between the two countries.

The U.S. has been supporting the airstrikes with intelligence and logistical support, and last week began refueling Saudi fighter jets. Administration officials say it is important to support Saudi Arabia. 

Earlier this week, a senior State Department official said the U.S. would try to ensure that a United Nations Security Council arms embargo against Houthi leadership is enforced. 

"We will be taking very careful look and examining very closely efforts to violate the embargo," senior State Department official Gerald Feierstein told the House Foreign Affairs Committee. 

The deepening of the conflict comes as the U.S. hopes to reach a deal with Iran to roll back its nuclear program in exchange for sanctions relief. Officials say U.S. support for the Saudi-led coalition has not affected the negotiations with Iran. 

The conflict also threatens to complicate U.S.'s relations with Iraq. Iraqi Prime Minister Haider al-Abadi, an Iran ally, criticized Saudi Arabia for its airstrike campaign during a visit to Washington this week. 

U.S. officials say they are unsure why Iran is making the brazen move. One theory they have floated is that the Saudi-led coalition has effectively blockaded any air routes into Yemen and there are no other ways to resupply the Houthis. 

Another theory is that Iran is trying to distract the coalition from another ship it has tried hard to conceal that is currently docked at Oman — a potential land route for smuggling arms into Yemen. 

Yet another theory is that Iran wants to force a confrontation with Saudi Arabia that it believes it will win, because Iran views the Saudi military as weak and suspects the U.S. lacks the willpower to support its Gulf ally. 

Earlier this week, Iranian Supreme Leader Ali Khamenei on Twitter taunted Saudi Arabia, calling its military puny and smaller than Israel's. He also said the air campaign was tantamount to genocide of innocent Yemeni civilians and that the U.S. would also fail in Yemen. 

U.S. officials say they hope the airstrikes will force Houthis to the negotiating table in order to restore stability in Yemen, where America faces a terrorist threat from al Qaeda in the Arabian Peninsula (AQAP). 

“We're assisting the Saudis to protect their own territory and to conduct operations that are designed to lead ultimately to a political settlement to Yemen,” said Defense Secretary Ashton Carter on Thursday. 

“That's good for the people of Yemen, first and foremost. It's good for Saudi Arabia that doesn't need this on its southern border.  And … it's good for us, among other reasons, because of AQAP's presence in Yemen. But for that to occur, it'll require more than military action,” he added. 

The Hill