The attack occurred on Friday afternoon in Let Pan Hla village in Singu township, about 65 kilometres north of Mandalay, the country's second-largest city, a spokesperson for the Mandalay People's Defence Force said.
The military did not comment on Saturday.
Myanmar has been in turmoil since the army seized power from the elected government of Aung San Suu Kyi on February 1, 2021, triggering widespread opposition.
After peaceful demonstrations were put down with lethal force, many opponents of military rule took up arms, and large parts of the country are now embroiled in conflict.
The military government has stepped up air strikes against the armed pro-democracy People's Defence Force and ethnic minority guerilla groups that have been fighting for greater autonomy for decades.
The two groups sometimes carry out joint operations against the army.
The resistance forces have no defence against air attacks.
The town of Singu was seized by the Mandalay People's Defence Force, which supports Myanmar's main opposition, in July last year.
The group's statement released Saturday on its Telegram social media channel said six children were among the 27 killed in the air strike that targeted crowded market stalls in Let Pan Hla village.
Osmond, the group's spokesperson, told The Associated Press that about 10 houses near the village market were destroyed by bombs.
"This air strike was not aimed at a military target, but rather at the market area, where civilians visit daily," Osmond said in a message.
The situation in the village could not be independently confirmed, with access to the internet and mobile phone service in the area mostly cut off.
The independent Myanmar Now online media reported that the death toll from the air strike that hit a tea shop near the village market had reached 30 and seven were in critical condition.
A report issued last month by Nyan Lynn Thit Analytica, a Myanmar research and advocacy organisation, said that since the military's 2021 takeover, 2224 civilians had been killed and 3466 wounded in 4157 air strikes.
