Myanmar's army has used air strikes against northern ethnic rebels, sources from both sides said Wednesday, in an escalation of a conflict that has raised doubts over the country's reform drive.

Fighting between the country's military, known as the 'Tatmadaw', and the armed wing of the KIO (Kachin Independence Organisation) has worsened in recent days as the army battled to regain one of its bases, a government negotiator involved in peace efforts told AFP.

"We heard the military used helicopters and training jets while trying to get their camp back," said Hla Maung Shwe, who is also an adviser to President Thein Sein.

He declined to specify how the aircraft were used, but a report on the military's Burmese language Myawaddy news website said a key base had been seized from the rebels on December 30 "with the help of air strikes in the region".

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.

KIO deputy chief of foreign affairs Colonel James Lum Dau said that the fighting in Kachin had become "more serious" since last week, adding that it was concentrated in an area about seven miles (11 kilometres) from the rebel headquarters in 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 whether the jets had been used to fire on the rebels.

A close observer of the situation, who asked not to be named, said there had been a "marked escalation" in fighting.

"They are firing a lot from helicopter gunships and using heavy artillery. It has been very close to the KIO headquarters," he said.

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

Myanmar's new quasi-civilian government has reached tentative ceasefires with most of the country's other major ethnic rebel groups, but several rounds of talks aimed at resolving the conflict in the country's far north have shown little tangible progress.

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.

The UN recently appealed to Myanmar to stop blocking aid to tens of thousands of displaced people in rebel-held territory in Kachin.

US 'deeply troubled' by Myanmar violence
Washington (AFP) Jan 03, 2013 - 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.