At least four Americans are being held in Yemen by rebels who toppled the US backed government, according to reports.
The Washington Post said attempts to free the Americans have failed.
The Americans are believed to be imprisoned in the capital Sanaa, which Saudi Arabia has repeatedly bombed in a campaign to oust the Shiite rebels, known as Houthis, from power.
The Houthis had cleared one of the prisoners for release, but the newspaper said members of the Houthi rebellion reversed that decision.
Three of the prisoners held private sector jobs, and the fourth holds dual US-Yemini citizenship. None is a US government employee, the report says.
The Post says the newspaper is withholding details about the four, at the request of relatives and US officials, who cited safety concerns.
Another American is being held in Yemen - Sharif Mobley, who is also in Houthi custody.
He has been held for more than five years on terrorism-related charges brought by the previous government, and his capture has been reported previously.
The US has limited contact with the Houthis.
The Post report said those most recently detained are among dozens of Americans who were unable to leave Yemen or who chose to remain in the country after the US closed its embassy.
The Houthis are part of a Shiite sect that receives backing from Iran.