Two captive Americans were freed on Wednesday in Yemen as more than 200 supporters of the country's Houthi rebels were allowed to return home, in an apparent swap involving Saudi Arabia and Oman.

The White House announced the release of US citizens Sandra Loli and Mikael Gidada, saying that the two had been held captive by the Iranian-backed Houthi rebels who control much of Yemen.

The rebels also sent back the remains of a third American, Bilal Fateen, the White House said, without describing the circumstances of how he died.

Robert O'Brien, the national security adviser, stopped short of describing an exchange of people but thanked Saudi King Salman and Omani Sultan Haitham Bin Tarik AlSaid "for their efforts to secure the release of our citizens".

President Donald Trump "continues to prioritise securing the release and repatriation of Americans held hostage abroad", O'Brien said in a statement.

"We will not rest until those held are home with their loved ones," he said.

The Houthis, who are backed by US arch-enemy Iran, said that 240 of their supporters returned to Yemen after being stranded in Oman, where they traveled two years ago for medical treatment.

"Thanks to God, about 240 fellow countrymen, who had been wounded and stranded, arrived in Sanaa on board two Omani planes," said Houthi spokesman Mohammed Abdel Salam.

"Among them were the wounded people who left for Muscat during the Sweden talks, and the United Nations did not return them according to the agreement," he said referring to the UN-sponsored peace process.

The Saudi-led military coalition in Yemen which supports the internationally recognised government did not comment on the release or whether there was a deal to exchange the 240 Houthi supporters.

But senior Houthi political official Mohamed Ali Al Houthi accused the coalition of refusing to allow the group to travel back earlier from Oman — which acts as a broker in a range of regional disputes.

"Today we were pleased to receive some wounded brothers who were stuck outside the country as a result of the brutal and continuous siege on our country. The coalition obstructed their exit and entry, one of its war crimes against Yemenis," Houthi said in a tweet.