YANGON: Kachin rebels on Monday said three civilians were killed and six wounded in the first direct attack by Myanmar forces on their northern stronghold during a recent upsurge of fighting.

Three shells landed in "the heart of Laiza", the Kachin Independence Army base near the Chinese border early Monday, Colonel James Lum Dau, spokesman for the KIA's political wing, told AFP.

"This is the first time they have directly bombarded Laiza," he said.

Lum Dau said the dead included a 15-year-old boy and a 76-year-old man.

AFP was unable to immediately contact the government for comment.

Growing fighting between the military and the KIA in recent weeks has overshadowed Myanmar's wider political reforms and cast doubt on a peace process seen as key to the country's emergence from decades of military rule.

The US and UN have condemned the army's use of air strikes in the state since December.

Tens of thousands of people have been displaced in Kachin -- which borders China -- since June 2011 when a 17-year ceasefire between the government and the KIA broke down. The number of casualties is unknown.

Myanmar's army and the rebels have traded claims over a helicopter crash last week which killed three army personnel. State media put the incident down to engine failure, rebutting KIA claims to have shot the craft down.

The intensification of the unrest in recent weeks is likely to further frustrate efforts to secure a peace deal in the region, observers have warned.

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