Military aircraft have been targeting rebel areas in Kachin in northern Myanmar over the last five days.
Video provided to the BBC by the Free Burma Rangers, a humanitarian group, showed attack helicopters firing at ground positions and jets flying close to the trenches of the Kachin Independence Army.
Fighting with the Kachin rebels resumed in 2011, after a truce lasting 17 years.
The BBC reports the presence of jets and attack helicopters in recent days has been confirmed by witnesses in the area.
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," he said.
A BBC correspondent said it appears that the military could be making preparations for a full-blown offensive against the rebel headquarters of Laiza.
