The United States said Wednesday it was "deeply troubled" by Myanmar army air strikes against northern ethnic rebels in the country's Kachin state.

Fighting between the country's military, known as the Tatmadaw, and the armed wing of the KIO -- the Kachin Independence Organization -- has worsened in recent days as the army battled to regain one of its bases.

"We note that the government did today admit that they have been using aerial weapons in Kachin State," State Department spokeswoman Victoria Nuland told reporters. "We're obviously deeply troubled by the increased violence."

"We are continuing to urge the Government of Burma and the Kachin Independence Organization to cease this conflict, to get to a real dialogue to address grievances as the Government of Burma has been able to do in virtually all of the other conflict areas," Nuland added.

Tens of thousands of people have been displaced since June 2011, when a 17-year ceasefire between the government and Kachin rebels broke down.

The rebels are calling for greater political rights and an end to alleged human rights abuses by the army.

Clashes in Kachin, along with communal unrest in western Rakhine state, have cast a shadow over Myanmar's widely praised emergence from decades of army rule.

US President Barack Obama visited Yangon in a historic visit last year, with Washington keen to expand its influence in a country where China has had almost unchallenged dominance.