Kachin rebels say shells fired by the Burmese military at their stronghold of Laiza has killed three civilians.
The deaths are the first reported in the town since the Burmese army began a new offensive last month, after a 17-year truce with the rebels ended in mid-2011.
Earlier this month, the government acknowledged that the military had carried out air attacks.
It said they were aimed at rebels who tried to block military supply routes.
The Kachin Independence Army, based in Kachin state in northern Burma, is the only major rebel group not to have reached a ceasefire with the government.
The KIA is seeking greater autonomy within Burma.
Tens of thousands of people have been displaced since the conflict reignited - some have fled to China's Yunnan province only to be forced back.
Laiza is the main rebel-held town. Reports said three shells struck the town centre on Monday morning. A 15-year-old boy was said to be among the dead and at least four other people were reported injured.
Burma's new military-backed civilian government - which has embarked on a series of reforms since elections in November 2010 - has pledged to resolve conflicts in border areas with ethnic minority groups.
But fighting in Kachin has intensified in recent weeks, with both the US and the UN condemning recent airstrikes.
Correspondents say the ongoing conflict raises questions over the extent of civilian government control over the armed forces, who ruled Burma for decades.