Four children were killed and five injured alongside their teacher as an explosion hit while they collected firewood in an area of Rakhine state beset by fighting between the military and Arakan Army (AA) rebels.
It was not immediately clear what caused the blast or who was behind it.
The conflict has seen scores of civilians killed, hundreds wounded and some 100,000 displaced in the past year as the AA fights for more autonomy for ethnic Rakhine Buddhists.
The attack happened Tuesday morning in Htaikhtoo Pauk village in Buthidaung township, deputy administrator Hla Shwe told AFP.
Local media posted a graphic video on Facebook showing people retrieving the victims' bodies and carrying the bloodied injured away as distressed crowds gathered.
"They were looking for firewood on the mountainside," Hla Shwe said by phone, adding the wounded had been taken to nearby hospitals in Buthidaung and Maungdaw.
He declined to say who he thought had been behind the attack.
Military spokesman Zaw Min Tun confirmed the incident and number of victims, accusing the AA of planting a landmine.
The rebels could not be reached for comment but one local village leader, who asked not to be named, told AFP the number of casualties and lack of blast crater made him doubt it had been a mine.
"Some people say a mine explosion, some say this was from heavy shelling."
The rebels have carried out a series of brazen kidnappings, bombings and raids against the military and local officials in recent months.
The army has hit back hard, deploying thousands of soldiers to the conflict-ridden region.
