A Burmese exile group is accusing government forces of systematically raping and sometimes killing women during its advance on strongholds of an ethnic rebel group.

In a press release issued late Tuesday, The Kachin Women's Association Thailand says at least 18 women and girls were gang-raped in a nine-day period after the offensive began on June 10. The group says four of the women were killed afterward, one in front of her husband, and another died of injuries while being raped.

The statement identifies specific villages where the rapes occurred and the Burmese army battalions responsible. It says the rapes do not appear to be random acts and charges that the soldiers are acting under orders. It also demands that Burma stop using rape as a weapon of war.

The charges cannot be independently confirmed because journalists cannot easily reach the areas where the fighting is taking place. The Burmese government has not commented on the treatment of civilians during he fighting.

Thousands of villagers have fled toward the Chinese border since the start of the fighting between government forces and rebels of the Kachin Independence Army. Witnesses say some refugees have been allowed into China, but many more have been turned back and have gone into hiding in the nearby forest.

The KIA, like several other ethnic militias in Burma, signed a cease-fire agreement with the central government several years ago. But those agreements began to break down in 2009, when Burma demanded that the militia groups come under central authority and serve as part of a national border guard. That prompted some militias to resume fighting.