Myanmar's army has launched a new offensive against ethnic Kachin rebels around their headquarters near the Chinese border, killing more than 20 people in a mortar attack and skirmishes that lasted the whole day, rebel officials said.

Most of the casualties occurred when government troops hit an officer training academy used by the rebels near the frontier city of Laiza, leaving 22 fighters dead and 15 more wounded, according to La Nan, a spokesman for the Kachin Independence Army.

La Nan said the army fired from a nearby hilltop about 50km outside Laiza. He said at least one Kachin villager was also killed in skirmishes in the area that began in the morning.

Myanmar's government could not immediately be reached for comment. Fighting between the army and Kachin fighters broke out in 2011, ending a truce between the two sides that had held for 17 years.

President Thein Sein's government has agreed to tentative peace deals with 14 Kachin factions since then, but has not been able to secure a deal with the fighters or broker a wider truce encompassing all armed groups.

After a major offensive near Laiza in December 2012, hostilities eased, but sporadic, low-level clashes have continued.

La Nan said the assault on the academy was an "unprovoked and a deliberate attack because [government soldiers] could clearly see our troops doing military training".

He said it was the highest casualty toll the rebels have suffered in a single attack in the past three years. The insurgents have lost about 300 fighters and another 400 have been wounded since 2011, he said.

Despite the bloodshed, La Nan said the rebels would continue to participate in future negotiations with the government.