WASHINGTON — Two Americans held hostage for months by rebels in Yemen were freed on Sunday and quickly flown to safety in nearby Oman, which helped the United States secure the release of the men, American officials said.

Along with the Americans, a British citizen and three Saudis were also freed by the Houthi rebels, who ousted the government of Yemen this year and are now facing a campaign of airstrikes led by Saudi Arabia and backed by the United States.

All six hostages were released in Sana, the capital of Yemen. American officials said on Sunday that they were still working to get details of what had led to the breakthrough in efforts to free the men.

The surprise release underscored how unusual it was for the United States and Britain to secure the freedom of citizens taken hostage by militants. Both countries refuse to pay ransom for their citizens, and as a result have struggled to bring home hostages who are held by Islamic extremist groups and other militants in the Middle East and South Asia.

“We welcome the release of two U.S. citizens who had been detained in Yemen since earlier this year,” said Ned Price, a spokesman for the National Security Council. “As we have informed their families, these individuals departed Yemen today and have since arrived in Oman.”

Successful negotiations to free American hostages have been rare. Among the successful cases were those of Sgt. Bowe Bergdahl, an American soldier taken prisoner by the Taliban who was released in 2014 in a prisoner exchange; and Theo Padnos, who was freed largely through the efforts of family and colleagues working with the government of Qatar.

But other hostages have not survived, including Americans and British citizens held by the Islamic State or Al Qaeda, some of whom were executed or inadvertently killed in American strikes or in rescue attempts.

The Houthis have detained hundreds of domestic political opponents, according to human rights groups. But unlike the Islamic State and Al Qaeda, the Houthis have not sought to indiscriminately abduct Westerners, and they appear to have treated those they detained more like prisoners of a foreign government than hostages. In June, the Houthis released Casey L. Coombs, an American journalist, after holding him for two weeks.

Amid the positive news on Sunday, there was also a grim reminder that other Americans were still being held in Yemen and elsewhere: An Obama administration official said the Houthis were still holding an American.

American officials identified the two men freed Sunday as Scott Darden, 45, and Sam Farran, 54. The two were caught in Yemen this spring when a Saudi-led bombing campaign began to dislodge the Houthis, a predominately Shiite Muslim movement that is viewed by the Sunni monarchies of the Persian Gulf as a proxy for Iran.

Mr. Darden and Mr. Farran were taken in late March by Yemen’s internal security forces, which by then were under the control of the Houthis, the American official said, speaking on the condition of anonymity because the administration had not officially named the men.

Mr. Darden was in Yemen helping oversee the import of relief supplies for Transoceanic Development, a logistics company based in New Orleans. Mr. Farran, a retired Marine from Michigan, was working as a security expert, the official said.

Britain’s foreign office would not identify the British citizen who was freed, saying only that British diplomats in Oman “met the flight from Yemen and are providing consular assistance. We are working closely with the Omani authorities.”

The release of the men was also a rare diplomatic breakthrough in Yemen’s six-month war, which has left more than 4,500 people dead.

Since March, the Saudi-led coalition, which includes nine other Arab states, has been conducting a bombing campaign and working with a loose confederation of Yemeni forces against the Houthis, who remain in power despite the airstrikes. Negotiations led by the United Nations have so far failed to halt the fighting.

But Oman has led what are widely considered to be the most promising and active efforts to date, hosting talks between the Houthis and their adversaries. Omani officials are said to have had a central role in the release of Mr. Coombs.

The state-run Oman News Agency said that the country had gotten involved in the efforts to free Mr. Darden and Mr. Farran after receiving “a request by the American government for help.” The news agency also confirmed the release of the three Saudis.

The White House praised Oman for its assistance, as did Secretary of State John Kerry in a separate statement released by the State Department.

“Since we first learned of their detention, the United States government has been in regular contact with their families and representatives and, in coordination with our international partners, we have worked tirelessly to secure their release,” said Mr. Price, the National Security Council spokesman.

Transoceanic, the employer of Mr. Darden, released a statement from its chief executive, Gregory Rusovich, saying, “We cannot begin to express the sense of joy and relief we feel with Scott’s release. He has been safely evacuated and will be reunited with his family very soon.”