Video footage shot by humanitarian group Free Burma Rangers shows attack helicopters firing on the ground and jets flying close to the trenches of the rebel Kachin Independence Army.
A government official told BBC the army had not informed them of any air attacks.
After a 17-year truce, fighting with the Kachin rebels resumed in 2011.
Though the presence of jets and attack helicopters was confirmed by witnesses, it was not clear how many casualties have been caused.
Many of the people who live in the conflict areas have already fled into camps, both in Kachin and across the border in China.
Asked to comment on the video, the director of the president's office, Zaw Htay said the situation was complex, and that the military had told them they were only using planes to re-supply its troops.
"The aircraft being used are K8 training aircraft not fighter jets - that is the information I got from the military. I have no information on the use of helicopters. There is a very difficult situation in Kachin state," he was quoted as saying.
The Free Burma Rangers, who reportedly filmed the footage while in rebel trenches, describes itself as "a multi-ethnic humanitarian service movement". The group works to provide aid in Myanmar's troubled border regions.
An estimated 75,000 people have been displaced by fighting in Kachin since 2011.
London: Military aircraft in Myanmar have been targeting rebel areas in the country's northern Kachin state since the last week of December 2012, BBC reported.
First Published: Wednesday, January 02, 2013, 22:43