The agreement in November "has not brought about an end to the ethnic cleansing of Tigrayans" in the disputed western part of Tigray, Human Rights Watch [HRW]'s Laetitia Bader said in a statement.
Security forces and militias from the Amhara region supported Ethiopia's army against rebels from neighboring Tigray during the two-year war in the country's north.
Western Tigray - fertile land under Tigray's authority but also claimed by Amharans as ancestrally theirs - was swiftly captured by federal and Amhara forces, and an interim administration took control.
A concerted campaign of "forced expulsions" began in earnest and did not let up despite the peace deal, HRW stated in a new report.
"Since the outbreak of armed conflict in Tigray in November 2020, Amhara security forces and interim authorities have carried out a campaign of ethnic cleansing against the Tigrayan population in Western Tigray, committing war crimes and crimes against humanity," HRW said.
It documented cases of arbitrary detention, torture and forced deportation from western Tigray, which stretches from the Tekeze River to the border with Sudan.
HRW said it interviewed 35 people by telephone between September and April including witnesses to the crimes, victims and staff from aid groups.
They said more than one thousand Tigrayans were detained at three sites in western Tigray "on the basis of their identity" before the Tigrayans were forcibly expelled in November 2022 or January 2023.
"As of March, militias in Western Tigray continued to threaten and harass Tigrayan civilians," HRW added.
The rights watchdog said it handed a summary of its preliminary findings to the Ethiopian government in May but had received no response.
"If the Ethiopian government is really serious about ensuring justice for abuses, then it should stop opposing independent investigations into the atrocities in Western Tigray and hold abusive officials and commanders to account," it said.
All sides to the two-year conflict were accused of possible war crimes and under the peace deal, the government is to create a "transitional justice" mechanism to identify and hold to account those responsible.