YANGON: Myanmar faced growing international pressure Thursday to halt air attacks on ethnic minority rebels in Kachin state, where an escalating conflict has overshadowed wider political reforms.

Fighting between the military and the armed wing of the Kachin Independence Organisation (KIO) in the far north of the country also known as Burma has worsened in recent days as the army battled to regain one of its bases.

"We're obviously deeply troubled by the increased violence," US State Department spokeswoman Victoria Nuland said in Washington late Wednesday, noting that Myanmar had admitted using aerial weapons in Kachin.

"We are continuing to urge the government of Burma and the Kachin Independence Organisation 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.

The military's Burmese-language Myawaddy news website has reported that a key base was seized from the rebels on December 30 "with the help of air strikes in the region".

Government peace negotiator Hla Maung Shwe, who is also an adviser to President Thein Sein, said military helicopters and "training jets" were believed to have been used in the operation.

UN Secretary General Ban Ki-moon added his voice to the calls for the Myanmar army to stop the air raids against Kachin rebels.

The UN chief "has taken serious note of the most recent reports indicating air strikes against targets in Kachin state", according to his spokesman Martin Nesirky.

"While details of these reports are still emerging and being closely followed, the secretary general calls upon the Myanmar authorities to desist from any action that could endanger the lives of civilians living in the area or further intensify the conflict in the region," Nesirky 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 Kachin Independence Army broke down.

The UN recently appealed to Myanmar to stop blocking aid to the displaced people in rebel-held territory in the state.

Civil war has gripped parts of Myanmar since independence from Britain in 1948.

The new quasi-civilian government 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 KIO accuses the government of pushing dialogue only on the basis of a ceasefire and troop withdrawals, neglecting to address longstanding demands for greater political rights.

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.

The fighting has persisted despite former general Thein Sein's order a year ago for the military to halt offensives against ethnic minority rebels, raising questions about his government's control of the armed forces.

The fighting in Kachin had become "more serious" since last week, according to KIO deputy chief of foreign affairs Colonel James Lum Dau, who said it was concentrated in an area about seven miles (11 kilometres) from the rebel headquarters at Laiza on the Chinese border.

"Before they (attacked) with helicopters, now they are using jets with rockets and bombs," he said.

AFP was not able to independently verify the claim.

Government spokesman Zaw Htay, who declined to give details of the latest fighting, said the Kachin rebels had not responded to an invitation for further dialogue.