A Myanmar military air strike killed 31 people at Mrauk-U hospital in Rakhine state on Wednesday evening.
Aid workers confirmed 68 others were wounded, and officials fear the death toll could rise further.
The attack comes amid an intensified military offensive ahead of elections starting December 28, as rebels resist junta control.
The Arakan Army (AA), an ethnic minority separatist force, controls almost all of Rakhine state and opposes the military’s polls.
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The AA previously allied with other rebel groups, but truces this year left it as the last active faction.
The military has blockaded Rakhine, causing widespread hunger and malnutrition, worsening the humanitarian crisis, according to the World Food Programme.
International observers, including the UN, criticized the elections, while China supports Myanmar, claiming stability restoration is needed in the country.