Joshua Holt, a U.S. citizen and Mormon missionary, is pictured in this still image taken from a selfie video which he posted on Facebook during a riot at the Helicoide detention center, Venezuela, obtained by Reuters May 16, 2018. MANDATORY CREDIT. Joshua Holt's Facebook Page/via REUTERS

WASHINGTON (Reuters) - The United States is worried about the safety of U.S. citizens held in Venezuela after a riot at a prison there on Wednesday and has appealed again for the release of Mormon missionary Joshua Holt, the State Department said on Thursday.

“We continue to have serious concerns about the safety and welfare of U.S. citizens who are being held there,” said State Department spokeswoman Heather Nauert. “The Venezuelan government is responsible for the safety of all detainees in its prison system, including U.S. citizens in detention.”

She said a senior State Department official had spoken to a top Venezuelan diplomat to seek the release of Holt on humanitarian grounds. Holt had posted information on Facebook about the riots.