The Health Ministry has not provided an official account of the cause of the fire.
The new ward, opened just three months ago, contained 70 beds, said two medical officials, who spoke on condition of anonymity in line with regulations.
Ammar al-Zamili, spokesman for the Dhi Qar health department, told local media there were at least 63 patients inside the ward when the fire began. Maj. Gen. Khalid Bohan, head of Iraq's civil defense, said in comments to the press that the building was constructed from flammable materials and prone to fire.
It was the second time a large fire has killed coronavirus patients in an Iraqi hospital this year. At least 82 people died at Ibn al-Khateeb hospital in Baghdad in April when an oxygen tank exploded, sparking the blaze.