Myanmar said Friday that air strikes targeting ethnic minority rebels in the northern state of Kachin were in self-defence, vowing "maximum restraint" in the face of growing international concern.

Fighting between the military, known as the Tatmadaw, and the Kachin Independence Army (KIA) in the far north of the country also known as Burma has worsened in recent days.

In a statement, Myanmar's reformist government accused the rebels of Kachin of blowing up railway tracks, roads and bridges, ambushing military columns and threatening local people.

"Due to these circumstances, the Tatmadaw had to take military action as self defence and in order to protect the safety of lives and properties of the people, safe and smooth transportation and peace and tranquility of the region," the statement said.

"Even though (the) Tatmadaw had to take this action, it has exercised with maximum restraint in using force," it added.

Tens of thousands of people have been displaced in the state of Kachin since June 2011, when a 17-year ceasefire between the government and the KIA broke down, prompting months of fierce fighting with gun and artillery fire.

The United States said Wednesday it was "deeply troubled" by the recent use of aid raids while UN chief Ban Ki-moon urged Myanmar "to desist from any action that could endanger the lives of civilians living in the area or further intensify the conflict in the region".

The Kachin clashes, along with communal unrest in the western state of Rakhine, have overshadowed dramatic political changes since Myanmar's widely praised emergence from decades of army rule in early 2011.

Myanmar has reached tentative ceasefires with most of the other major ethnic rebel groups, but several rounds of talks with the Kachin have shown little tangible progress.

The Kachin accuse the government of pushing dialogue only on the basis of a ceasefire and troop withdrawals, neglecting to address longstanding demands for greater political rights and complaints about Chinese-led construction of dams and other projects in the resource-rich region.

China, traditionally a key backer of the Myanmar regime, said Friday it had made a diplomatic complaint to Myanmar after three bombs landed on its territory on Sunday evening but caused no casualties.