The United States on Thursday called on Iranian-backed Huthi rebels who control Yemen's capital Sanaa to release an unspecified number of local residents who work for the US embassy.

The compound that used to serve as the embassy -- operations were moved to Saudi Arabia years ago because of Yemen's civil war -- has been breached, a State Department spokesperson said.

Most of the detainees have in fact been released but the rebels continue to hold Yemeni employees of the embassy, the spokesperson said.

"We are concerned that Yemeni staff of the US Embassy in Sanaa continue to be detained without explanation and we call for their immediate release," the spokesperson said.

The spokesperson also called on the Huthis to "immediately vacate" the embassy compound and return all seized property.

At a briefing Tuesday, State Department spokesman Ned Price declined to say how many local employees had been detained or why.

"We are committed to ensuring the safety of those who serve the US government overseas, and that is why we are so actively engaged on this matter, including through our international partners," Price said.

The United States transferred embassy operations to Riyadh in 2015 because of Yemen's civil war, which pits the Huthi rebels against the US- and Saudi-backed government.

The war has created what the United Nations has described as the world's worst humanitarian crisis.

About 80 percent of Yemen's 30 million people rely on aid to survive.

Upon taking office in January, President Joe Biden promised a break with former president Donald Trump's staunch support for the Saudis including the kingdom's devastating air campaign in Yemen, which targets the Huthis but has led to wide accounts of civilian casualties.