At least four U.S. citizens held prisoner by Houthi rebels in Yemen

SANAA, Yemen, June 1 (UPI) -- Multiple American citizens are being held prisoner by the Houthi rebel group in Yemen, as attempts to secure them have failed.

At least four U.S. citizens, including one believed to hold a dual U.S. and Yemeni citizenship, have been taken and are being held at a prison in Yemen's capital Sanaa, according to The Washington Post. Sanaa has been subject to an aerial bombing campaign led by Saudi Arabia since Houthis came to power earlier this year.

Attempts to secure the release of the Americans by the Department of State have been complicated because the Houthi rebels and the United States are on opposite sides in Yemen's conflict, according to The New York Times. The White House has publicly supported Saudi Arabia and its coalition against the rebels, to the disdain of the Houthis.

One of the imprisoned Americans was cleared for release recently, only for members of the Houthi rebellion to reverse that decision "because he overstayed his visa," an anonymous U.S. official told The Washington Post.

A fifth American citizen, Sharif Mobley, has been held in Yemen for more than five years on terrorism charges.

The Saudi-led coalition against Houthi rebels began its aerial assault late March. A five-day ceasefire was enacted in May to replenish food, water and medical supplies in Yemen.

More than 1,600 people have been reportedly killed in the Saudi-led airstrikes in Yemen.

According to the United Nations, up to 300,000 Yemenis have fled their homes and 12 million lack sufficient food, clean water, fuel or basic medical care since the beginning of hostilities.