Kachin rebels in Myanmar said three civilians were killed and six wounded on Monday in the first government attack on their stronghold, as fighting escalated in the country's last active civil war. Three shells landed in the centre of Laiza, a town on the northern border with China that serves as headquarters for the Kachin Independence Army, said Colonel James Lum Dau, spokesman for the KIA's political wing. "This is the first time they have directly bombarded Laiza," he told AFP. The KIA said the victims of the early morning strike included a 15-year-old boy and a 76-year-old man. A video contributor to AFP saw three bodies after the shells landed near one of the town's main roads, in an area where people were going about their daily chores, and said the attack had left residents fearful and weeping. He said that two small children were among the injured, and that the casualties were transported to Laiza hospital on makeshift stretchers. The shelling came after the US and UN condemned the army's use of air strikes in an upsurge of fighting since December that has raised questions over the government's commitment to reform after a transition from military rule. Myanmar presidential spokesman Ye Htut told AFP that he had received no information about the attack and said the army did not "intentionally" target civilians. "We do not know where the civilians were," he said. "One thing for sure is we do not fire aiming to the civilian targets." La Rip, from the Relief Action Network for IDP (internally displaced persons) and Refugees, said people from the affected areas were taking shelter close to the border. "People are scared. At the moment it has stopped, but I'm afraid the shelling will continue, they are not targeting only military targets," the Kachin aid worker told AFP. Around 20,000 residents and 15,000 displaced people are thought to be in Laiza, he said, adding that there was "nowhere to go" except to China, which in August pushed several thousand refugees back into Myanmar. Tens of thousands of people have been displaced in Kachin state since June 2011 when a 17-year ceasefire between the government and the KIA broke down. The total number of casualties is unknown. An increase in fighting between the military and the KIA in recent weeks has overshadowed Myanmar's wider political reforms and cast doubt over a peace process seen as key to the country's emergence from decades of military rule. 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. The rebels accuse the government of pushing for a dialogue based only on a ceasefire and troop withdrawals, and failing to address longstanding demands for greater political rights. While the Myanmar army has used helicopters in the conflict in the past, the use of fighter jets in recent weeks is seen by observers as part of a marked escalation in the fighting. 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 it down. President Thein Sein defended the army's response to the Kachin rebellion in comments reported in state media on Friday, saying the Tatmadaw -- Myanmar's army -- had done everything possible "to make positive contributions to the peace process". Some experts have however raised questions over the level of control Thein Sein, a former general, exerts over army units in Kachin after an order to end military offensives in December 2011 was apparently ignored. He since said that the army only acted in self defence, laying the blame for the unrest on the rebels.