Myanmar's army has fired artillery shells into the heart of a town used by ethnic Kachin rebels as their headquarters, killing three civilians and wounding six others in the latest escalation of the conflict, the rebels have said.

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 [KIA], James Lum Dau, rebel spokesman, said on Monday.

"This is the first time they have directly bombarded Laiza," he told the AFP news agency.

The deaths, which included a 15-year-old boy, were the first war casualties reported in Laiza since the two sides resumed fighting in mid-2011 after nearly two decades of peace.

Al Jazeera's Wayne Hay, reporting from neighbouring Thailand, said the latest artillery fire followed a series of airstrikes in the region in recent weeks.

"The fear among some was that government troops would launch an attempt to seize Laiza from the rebels who are fighting for greater autonomy," Hay said.

A video obtained by Al Jazeera showed civilians running for their life and fatalities after the shells landed in private homes. The video also showed a wounded child.

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.

Ye Htut, Myanmar's presidential spokesman, told AFP he had received no information about the attack and said the army did not "intentionally" target civilians.

Peace talks

La Rip, from the Relief Action Network for refugees, said people from the affected areas were taking shelter close to the border.

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.

President Thein Sein defended the army's response to the Kachin rebellion in comments reported in state media on Friday, saying the army had done everything possible "to make positive contributions to the peace process".

Following the attacks, neighbouring China announced it was stepping up security on its border with Myanmar.

"We hope the Myanmar government can ease tension through peaceful negotiations with relevant parties, prevent conflicts from escalating, and properly resolve disputes," Hong Lei, the foreign ministry spokesman, said.

"China has taken necessary measures to enhance border control to protect safety of Chinese citizens and their properties at the border," Hong said.

While China has strong business and trade ties with Myanmar, it has long looked with wariness at its poor and unstable southern neighbour and has repeatedly called on the country to ensure stability along the vast and remote border.

China had forcibly returned scores of ethnic Kachin who have fled Myanmar, a human rights group said last year.

Source: Al Jazeera and agencies