In a statement issued by its high command, the Oromo Liberation Army [OLA] said that after the first round of talks concluded on May 3, Ethiopian forces launched a "broad offensive."
"This move starkly contradicts the understanding that de-escalation should be prioritized during the negotiating processes," it said, accusing the army of forcing farmers to join local militias and supply food to its troops.
The government and the OLA held negotiations for a week in Zanzibar in a bid to end a decades-long conflict that, in recent years, has left hundreds dead and displaced tens of thousands in Ethiopia's most populous region.
Both sides said at the time that they had agreed to continue talking.
The OLA is an outlawed splinter group of the Oromo Liberation Front (OLF), a formerly banned opposition party that returned from exile after Abiy took office in 2018.
Oromiya, which surrounds the capital Addis Ababa, is home to Ethiopia's largest ethnic group and over a third of the country's 110 million people.
It has experienced unrest for decades, rooted in grievances about alleged marginalization and neglect by the federal government.
The violence there is a major security challenge for Prime Minister Abiy Ahmed, whose government signed a peace deal last November to end a two-year civil war in the northern Tigray region.
Spokespeople for Ethiopia's federal government, the army and Oromiya's regional administration did not immediately respond to requests for comment.
Reuters was not able to independently verify the OLA's claims.