The U.S. reportedly has taken on a greater role in the Saudi-led campaign of airstrikes against Shiite rebels in Yemen amid doubts about whether Riyadh can achieve its objectives. 

The Wall Street Journal reported Sunday that the U.S. Navy has stepped up patrols looking for weapons that American and Arab officials believe have been sent to the rebel forces, known as Houthis, by Iran. On one occasion, the paper reports, U.S. sailors boarded a Panama-flagged freighter in the Red Sea that was suspected of carrying armaments. The search turned up nothing. 

The paper also reported that the Obama administration authorized Pentagon war planners to cross-check a list of targets provided by the Saudis against U.S. intelligence and provide feedback ahead of the first airstrikes last month. 

Two-and-a-half weeks of aerial and naval bombardment have done little to slow the Houthi advance in Yemen, though it has prevented the rebels from controlling the port city of Aden. The Journal reports that the relative lack of success has caused Washington to call on the Saudis to revisit the goals of their campaign. 

Saudi officials tell the paper that their goal is to degrade the capabilities of the Houthis and restore Yemen's deposed president, Abed Rabbo Mansour Hadi, to his office. Hadi is currently in Saudi Arabia after being forced to flee by the Houthis just prior to the start of airstrikes. 

According to the Journal, however, the Obama administration wants Saudi Arabia to focus on stopping the Houthi advances, achieving a military impasse that would force all parties to the negotiating table. The paper reports that the Obama administration has embraced this strategy due in part to fears of more direct action by Iran and concern that civilian casualties will undermine any popular support for the Saudi-led campaign. 

United Nations officials say that at least 648 civilians have been killed since the airstrikes began March 25. The Saudis have blamed the Houthis and their Yemeni allies for the civilian deaths and say they are doing their best to limit them. 

On Sunday, Saudi Arabia's foreign minister refused calls from Tehran to stop its aerial assault. Iran supports the Houthis, who are fellow Shiites, but both Tehran and the rebels deny it arms them.

Prince Saud al-Faisal said his country is not at war with Iran, but charged Tehran with fueling the cycle of violence in Yemen. Riyadh and Tehran are longtime regional rivals, and also back opposite sides in Syria's civil war.

"Iran is not in charge of Yemen," al-Faisal said during a press conference Sunday in Riyadh alongside his French counterpart, Foreign Minister Laurent Fabius.

Writing in an opinion piece in The New York Times, Hadi on Sunday called the Houthis "puppets of the Iranian government" and declared: "If the Houthis do not withdraw and disarm their militia and rejoin the political dialogue, we will continue to urge the coalition to continue its military campaign against them."

Hadi called for "continued international support to ensure military might on the battlefield" and warned that oil shipments through the Red Sea will be endangered if the Houthis are not stopped.

Saudi military spokesman Ahmed Asiri said during a briefing Sunday that airstrikes hit an airport in the Houthi stronghold of Saada, as well as areas used by the Houthis and their allies in Shabwa, Sanaa, Taiz and Aden.

He added that the Saudi government is coordinating with some of the tribes in Yemen.

Sunday's airstrikes in the oil- and gas-rich north-central province of Marib hit groups of Houthi and allied fighters, Yemeni security officials told the Associated Press. All officials spoke on condition of anonymity because they were not authorized to speak to journalists.

Sheikh Saleh al-Anjaf, spokesman for an alliance of Marib-based tribes, said Houthis and Saleh loyalists tried to advance from the ancient ruins of Sirwah but were pushed back following a fierce battle with tribesmen.

Al-Anjaf said six tribesmen were killed and seven wounded in the fighting and that the tribe took six Houthi fighters hostage. The Houthis did not announce any casualties, and Houthi officials could not be reached for comment.

The Associated Press contributed to this report.

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