YANGON (Reuters) - Myanmar police said on Saturday they had opened a murder investigation into the killing of two men that residents say appeared to have been shot in the head after they were detained by soldiers in the conflict-torn north of the country.

Residents say the men were ethnic Kachin farmers who were living in camps for people displaced by conflict in Maing Hkawng, a village about 30 km (20 miles) from Myanmar’s border with China.

More than 100,000 have been forced from their villages since a ceasefire between Myanmar’s army and the Kachin Independence Army (KIA) broke down in 2011.

The northern conflict is separate from the Rohingya crisis stemming from western Rakhine state but the military’s campaign has similarly been beset by allegations of abuses against civilians.

Ethnic Kachin men Hpaugan Yaw, 65, and Nhkum Naw San, 35, had been missing since they were taken away by the military on Jan. 31, according to residents and campaign group Fortify Rights, which interviewed two witnesses to their detention.

A shallow grave containing two bodies was discovered on Thursday by a civilian search party in a forest near Maing Khawng, in Kachin state’s Mansi township. Authorities visited the grave on Friday and a doctor conducted an autopsy on the bodies.

“The case has been filed at Mansi police station under section 302 (of Myanmar’s Penal Code), a murder charge,” said Thura Myo Naung, a spokesman for police in Kachin state who declined to comment further other than to say police were investigating.

The Ministry of Home Affairs said in a statement posted on its Facebook page the bodies had “penetration wounds on the left cheek” and the backs of their heads had “exploded”.

The men went missing while herding buffalo, according to a missing persons complaint filed on Feb. 5, said the statement, which did not mention the alleged involvement of soldiers.

Military spokesman Major General Tun Tun Nyi declined to comment on the case.

Fortify Rights had raised concerns about the two men last month and cited two witnesses who said they saw soldiers detain them. One witness said soldiers tried to put KIA uniforms on the men, according to the group, which kept the witnesses anonymous.

Aung Myu San, a Kachin youth leader in Maing Hkawng, said the doctor’s autopsy report had not yet been made available but he had seen the bodies along with other residents.

“When we personally went there to look, we saw the situation and we thought it looked like a gunshot to the mouth because the back part of the head is totally gone,” he said.

Myanmar’s security forces are also accused of rape, murder and arson of Muslim Rohingya in a campaign launched in response to insurgent attacks in August in the western Rakhine state, prompting hundreds of thousands to flee to neighboring Bangladesh.

Myanmar has asked for “clear evidence” of atrocities in Rakhine, but the U.N. human rights chief Zeid Ra’ad al-Hussein called on Friday for Myanmar to be referred to the International Criminal Court for what he said may be “acts of genocide” against the stateless Rohingya minority.