GOMA, Congo — With Congo’s army diverted to fighting a new rebel group in eastern Congo, new militia groups have arisen and older ones are reasserting themselves, killing hundreds of defenseless civilians, the British charity Oxfam said Tuesday.

Underscoring the severity of the situation, UN humanitarian chief Valerie Amos arrived in Goma on Tuesday and is to visit some of the 280,000 people who have fled their homes since mutinying soldiers launched the M23 rebellion in April. But security is so poor that Amos was forced to cancel planned trips to the mining town of Walikale and the seat of the rebellion in Rutshuru, 50 miles north of Goma.

As the 150,000-strong Congolese army and 20,000 UN peacekeepers have redeployed against M23 rebels in North Kivu province, fighting has spread to villages and towns, with the combatants often aiming to gain control of mines.

“We support the efforts of the [Democratic Republic of Congo] and we urge all the states in the region, including Rwanda, to work together to cut off support for the rebels in the M23, to disarm them, and to bring their leaders to justice,” US Secretary of State Hillary Rodham Clinton said.

Responding to the escalating crisis in the mineral-rich area, regional leaders met in Uganda on Tuesday, where they may seek a change in the mandate of the UN peacekeeping mission. The option is one of three under consideration as presidents forming the 11-nation International Conference on the Great Lakes Region pursue a solution to the crisis.

Other options include incorporating a neutral force drawn from around Africa into the UN peacekeeping mission in Congo, an idea that appears to be favored by the Congolese, or establishing a force from regional armies, a more desirable situation for Uganda and Rwanda, according to officials monitoring deliberations.