A group of women U.S. senators have accused Burma's military of using rape as a weapon of war and are urging Secretary of State Hillary Clinton to pressure the Burmese government to halt the practice.

In a letter Wednesday, the bi-partisan group asked Secretary Clinton to support establishment of an international commission of inquiry into war crimes and crimes against humanity in Burma.

The letter cites a report saying that 18 ethnic Kachin women were gang-raped by government troops during the fighting that ended a 17-year cease-fire, and says there are similar reports from neighboring Shan state, where hostilities have also resumed.

Democratic Senator Barbara Boxer and Republicans Kay Bailey Hutchison are among the 13 signatories.

The lawmakers also quote Burma's opposition leader Aung San Suu Kyi, who said that rape is used in her country as a weapon by armed forces to intimidate ethnic groups and divide the country. The Nobel Prize laureate made the statement in a video-taped message to a Nobel Women's conference in May.

Human rights groups have repeatedly condemned Burma's authorities for abusing rights of its minority groups. Amnesty International says government troops for decades have used rape, torture, forced relocation and killing to intimidate ethnic minorities into ending their campaign for autonomy.

Larger groups such as Shan, Karen and Kachin people have been especially targeted.

The United States last week appointed its first special envoy to Burma in a move to coordinate efforts with U.S. allies to get Burma's new government to implement democratic reforms. Derek Mitchell will lead U.S. efforts to improve what he called the southeast Asian nation's abysmal human rights record during four decades of military rule.

A new, nominally civilian government was elected last November in a vote orchestrated by the military.