Houthi supporters brandish weapons and shout slogans during a rally Saturday protesting a U.N. roadmap for the Yemen conflict. (Yahya Arhab/European Pressphoto Agency)

An American held by Houthi rebels in Yemen for a year and a half was freed Sunday after negotiations involving Secretary of State John F. Kerry and the Sultan of Oman, State Department officials said.

Wallead Yusuf Pitts Luqman had been in captivity since April, 2015, when he was taken off a bus while he and his family were trying to leave Yemen for Saudi Arabia, according to a posting made by his wife on his Facebook page. He had been teaching English in Yemen, one of the poorest countries in the Middle East.

Several other Americans have previously been released by the rebels, and officials believe others are still being held. Luqman’s detention was not widely known until just two weeks ago, when his wife, Jihan Wallead, posted a plea for his release.

“They had no reason to take him and there are no charges,” she wrote. “After months of not knowing his condition or where he was, I finally received proof of life this year.”

She said he had been taken away by the Houthis’ Political Security Organization, one of Yemen’s main internal security and intelligence agencies. The Shiite Houthis have been in charge of the capital of Sanaa and other areas since 2014. Thousands have died in the war between forces allied with the Houthis and those loyal to the internationally backed government. Many civilians have been among the casualties in airstrikes against the Houthis by a coalition of Arab countries led by Saudi Arabia in support of Yemen’s government.

Kerry has appealed to Sultan Qaboos bin Said Al Said for his help in the other cases involving Americans held in Yemen. About two weeks ago, Kerry met with Luqman’s family, and he called Luqman’s wife Sunday to tell her that her husband had been flown to Oman, according to a senior State Department official.

In a statement, Kerry hailed news of his return, first reported by the Omani state news agency ONA. He thanked both the sultan and the Omani government for their help.

“We also recognize this positive gesture by the Houthis,” Kerry said. He called for the release of other “individuals unjustly held in Yemen.”

The United Nations has attempted to put a cease-fire in place and resume peace talks, but their efforts so far have failed.