A U.N. rights expert has expressed grave concern over reports that Myanmar's military has used heavy weapons and conducted aerial bombings in civilian areas as fighting against ethnic rebels in northern Kachin state has escalated sharply.

While the world focuses on the plight of Myanmar's Rohingya Muslims, a civil war has flared against the Kachin Independence Army, with thousands of civilians displaced from their villages near the Chinese border, according to the United Nations.

The U.N. special rapporteur on the situation of human rights in Myanmar, Yanghee Lee, said the situation in Kachin state "is wholly unacceptable, and must stop immediately."

She said civilians must never be subjected to violence during conflict.

The Kachin, who are mostly Christian, have fought for greater autonomy in the predominantly Buddhist nation since 1961.