Fighters from the Kachin ethnic minority have accused the government of Myanmar of launching an attack on communities in the north of the country.
The reported Sunday attacks come days after a ceasefire pledge by the country's reformist government.
"They're still fighting," said James Lum Dau, the Thailand-based spokesman for the Kachin Independence Organisation (KIO), the political wing of the rebel Kachin Independence Army (KIA).
He said the military was battling to retake control of a strategically important hilltop several kilometres away from the KIA headquarters in Laiza using artillery shells and ground forces.
"They want to take all their artillery on the hilltop and then they will immediately do something to occupy Laiza," he said.
Witnesses reported seeing hundreds of government troops involved in the operation.
The Myanmar government announced on Friday that it would end a military offensive against the Kachin fighters with effect from Saturday morning, but within hours the fighters reported coming under fire again.
The government pledge came amid growing international concern about the use of air strikes in the conflict.
Since the ceasefire vow, however, there have been no reports of further air raids.
Tens of thousands of people have been displaced in the state since June 2011, when a 17-year ceasefire between the government and the KIA broke down.
The total number of casualties is unknown, but the government said Friday that 35 soldiers had been killed and 190 injured in a series of ambushes by the fighters since 2011.
Myanmar's quasi-civilian government has reached tentative ceasefires with a number of major ethnic rebel groups since taking power in early 2011, but several rounds of talks with Kachin fighters have shown little progress.
The KIO accuses the government of pushing dialogue only on the basis of a ceasefire and troop withdrawals, neglecting to address longstanding demands for greater political rights.