More than 70 people were injured, they said.
The hospital in Rakhine's Mrauk U township was struck late on Wednesday by bombs dropped by a military aircraft, said Khine Thu Kha, a spokesman for the Arakan Army.
"The Mrauk U General Hospital was completely destroyed," Khine Thu Kha told Reuters. "The high number of casualties occurred because the hospital took a direct hit."
A junta spokesman did not respond to calls for comment.
Myanmar has been gripped by conflict since the military suppressed protests against a 2021 coup that unseated the elected government led by Nobel laureate Aung San Suu Kyi.
The 300-bed hospital was overflowing with patients at the time of the strike, said aid worker Wai Hun Aung, as most healthcare services across swathes of Rakhine state have been suspended amid the ongoing fighting.
The UN human rights chief Volker Turk said such attacks may amount to a war crime and called for an investigation.
"I am appalled and condemn in strongest possible terms the strikes on Rakhine hospital," he stated in a post on X, adding he is concerned about new waves of acute violence and fear in Myanmar.
On Thursday morning, the facility lay in complete ruins, with a collapsed roof, shattered columns and beams, and the bodies of victims laid out on the ground, according to images shared by Wai Hun Aung that he also posted on social media.
Reuters could not immediately verify the images.
"The remaining patients have been moved to a safe location," he told Reuters.
Soon after he heard the sound of explosions, a 23-year-old resident of Mrauk U said he rushed to the site.
"When I arrived, the hospital was on fire," he said, asking not to be named because of security concerns. "I saw many bodies lying around and many injured people."
The junta, which has the only air force in Myanmar, has been increasingly using air strikes to hit targets inside rebel-held areas.
From January to late November this year, the junta conducted 2165 air strikes, compared to 1716 such incidents during the whole of 2024, according to the Armed Conflict Location & Event Data Project.
Mrauk U township, located in the north of Rakhine state, has been under the control of the Arakan Army since last year and there has been no recent fighting in the area, Khine Thu Kha said.
