At least four Americans are being held in Yemen by rebels who toppled the US-backed government, the Washington Post reported late on Friday.

Citing unnamed sources, the report said attempts to free the Americans have failed. The Americans are believed to be imprisoned in the capital Sana’a, which Saudi Arabia has repeatedly bombed in a campaign to oust the rebels, known as Houthis, from power.

The Houthis had cleared one of the prisoners for release, but the report said members of the Houthi rebellion reversed that decision.

Three of the prisoners hold private sector jobs, and the fourth holds dual US-Yemini citizenship. None is a US government employee, the report said.

The Post report said the newspaper was withholding details about the four at the request of relatives and US officials, who cited safety concerns.

One more American is being held in Yemen. Sharif Mobley is also in Houthi custody. He has been held for more than five years on terrorism-related charges brought by the previous government.

Few details were available. The US has limited contact with the Houthis.

The Post report said those most recently detained were among dozens of Americans who were unable to leave Yemen or who chose to remain in the country after the US closed its embassy.