Rohingya insurgents carried out at least one brutal massacre in Rakhine state, the organisation says.
Amnesty has investigated the alleged killings of minority Hindus on 25 August last year and concluded that Rohingya militants were involved.
The Rohingya are an ethnic group, the majority of whom are Muslim, who have lived for centuries in the mainly Buddhist country of Myanmar, also known as Burma.
Minority Hindus also live alongside them.
Claims that Arakan Rohingya Salvation Army (ARSA) had carried out a massacre in a village called Ah Nauk Kha Maung Sei first surfaced hours after it took place.
The claims came from government and security forces.
On the same day, Rohingya militants attacked 30 police posts and an army base in Rakhine, a move that provoked a counter-offensive by the army that eventually drove nearly 700,000 Rohingya civilians into Bangladesh.
Mass graves containing dozens of bodies were said to have been found at the time. About 100 Hindus were also reported missing.
In December, Myanmar’s military flew reporters to a spot in the north of Rakhine where they showed the bodies of Hindus in mass graves.
The reports proved hard to confirm initially as survivors who reached Bangladesh gave conflicting accounts. Some blamed ethnic Rakhine villagers.
Amnesty International said it had carried out “a careful review of evidence” including a study of the statements of witnesses and the analysis of images by forensic pathologists.
One of the women they spoke to, 22-year-old Bina Bala, told Amnesty: “They [ARSA] came to our house.
“[The men] held knives and long iron rods. They tied our hands behind our backs and blindfolded us.
“I asked what they were doing. One of them replied, ‘You and [ethnic] Rakhine are the same, you have a different religion, you can’t live here.’ They asked what belongings we had, then they beat us.”
Tirana Hassan, Amnesty’s crisis response director, said: “Our latest investigation on the ground sheds much-needed light on the largely under-reported human rights abuses by ARSA during northern Rakhine state’s unspeakably dark recent history.
“Accountability for these atrocities is every bit as crucial as it is for the crimes against humanity carried out by Myanmar’s security forces.”
Doctors Without Borders estimates at least 6,700 Rohingya were killed during the first month of the conflict with the army in Rakhine.
ARSA has not yet responded to the Amnesty International report.
It is unclear why the alleged attack was carried out, but some suspect the militants believed the Hindu community sympathised with the predominantly Buddhist government’s anti-Rohingya stance.
More from Rohingyas
Boatloads of Rohingya refugees ‘risking death at sea’
BAFTA award for Sky News’ coverage of Myanmar’s Rohingya crisis
UK pledges £70m Rohingya aid ahead of rainy season
Doctors prepare for spike in Rohingya women giving birth after sexual attacks
MPs urge Leeds United owner Andrea Radrizzani to cancel Myanmar tour
Leeds United accused of ‘assisting a brutal regime’ with Myanmar tour
Amnesty International also claimed to have uncovered other killings of Hindus.
David Mathieson, a former human rights researcher who is now an independent analyst based in Myanmar, said that while the Amnesty International findings confirm reports of ARSA’s brutality, it did not justify the army’s systematic driving out of the Rohingya people.