A government official admitted that the Myanmar military carried out airstrikes against ethnic rebels in Kachin state after previous government reports denied the offensive.

In an interview with Voice of America's Burmese Service, an unnamed government representative said jets were used against fighters of the Kachin Independence Organization.

Earlier reports, including by the BBC, said the director of the president's office, Zaw Htay, claimed that only training aircraft were used in the area and only to resupply soldiers on the ground.

However, video footage shot by the humanitarian agency Free Burma Rangers and shown by the BBC on its website allegedly shows a military plane firing rockets at rebels in trenches.

"The aircraft being used are K8 training aircraft, not fighter jets, that's the information I got from the military," the BBC quotes Zaw as saying.

But the VOA report said the Myanmar government backtracked on its previous denial. The station's source said the army used airstrikes to help regain a base during an increase in fighting that started last week near rebel headquarters in Laiza, close to the Chinese border.

The escalation in fighting in the northern state could be a setback to the government's ongoing reforms to move from a previous military dictatorship to a more open democratic society -- and the lifting of more international economic sanctions.

In particular, the increased fighting and use of aircraft could be a blow to the reform plans of President Thein Sein, a senior junta leader turned civilian politician who won a national election in late 2010.

The BBC said the video footage suggests that the army is going beyond Thein Sein's public instructions to fight only in self-defense.

It also raises questions about how much authority Thein Sein, a 67-year-old ex-general has over the military, which is guaranteed 25 percent of seats in Parliament and continues to be a major player in civilian political life, the BBC said.

Despite the move to a more open and democratic society, the government continues its long-running fight with rebels in several states including Kachin, Karen, Shan and Mon.

The conflicts have been running for most of the years since Myanmar -- formerly called Burma -- gained independence from the British in 1948.

Many of the rebel groups are fighting for more autonomy from the central government and a greater share in the exploitation of natural resources to raise living standards in their regions.

In Kachin state, tens of thousands of people have been displaced when a 17-year cease-fire collapsed in June 2011.

A report by the Irrawaddy news website claims the military also is using helicopter gunships.

Hla Seng, a soldier from the All Burma Students' Democratic Front, an armed group fighting alongside Kachin rebels, said four helicopters, including Mi-24s, fired on KIA bases.

"They (the army) have been attacking us non-stop by using the planes for six days," Irrawaddy quotes Hla as saying.

"Now, they're heating up the war by using jet fighters, helicopter gunships, artillery weapons and chemical weapons," he told he Irrawaddy in a phone call from the region.

Lamai Gum Ja, a Kachin national who heads the intermediary civilian Kachin Peace-Talk Creation Group, condemned the use of airstrikes.

"I think it's inappropriate," he said. "It can harm the peacemaking process."