The civilians, mostly pastoralists from the Tutsi tribe who cherish their cattle, were warned last week by UN peacekeepers to flee the town of Kirolirwe, which serves as a military base for renegade Tutsi General Laurent Nkunda.

The army threw more than 20,000 soldiers into a major offensive a week ago to forcibly disarm Nkunda's estimated 4,000 fighters and end their three-year insurgency.

Government forces on Monday advanced slowly on Kirolirwe, blasting rebel positions with rockets and artillery in a hilltop-to-hilltop push into Nkunda's rugged fiefdom. Kirolirwe lies 35 km (22 miles) northwest of the provincial capital Goma.

"They are unwilling to leave there," MONUC military spokesman in North Kivu, Major P.K. Tiwari, told Reuters.

"The reason they cite is that they have cattle and cannot move upwards because there's no grazing ground. They cannot come down, because there's fighting there, and there's nowhere else they can move."

Some aid workers accuse Nkunda, a Tutsi, of manipulating Tutsi refugees for his military goals.

In addition to a permanent population of around 40,000, some 15,000 refugees live in camps in Kirolirwe, most of them women and children belonging to the Tutsi minority.

Many fled there believing that Nkunda would protect them after his fighters abandoned a peace deal in August and withdrew from mixed army brigades.

"The two main risks are that they are hit by artillery, or they are used as human shields," said the MONUC spokeswoman in North Kivu, Sylvie Van Den Wildenberg.

Tiwari said the town, which lies on the main road to Nkunda's headquarters in Kichanga 10 km (6 miles) further north, is within range of government artillery, but MONUC had received assurances the army would not immediately attack.

MONUC, which has a mobile base in the town and is mandated to protect civilians, would not abandon the town while the population remained. "We can't leave these people there. It's not correct," Tiwari said.

More than 400,000 people have fled fighting in North Kivu over the past year amid a crisis rooted in neighbouring Rwanda's 1994 genocide, during which Hutu soldiers and Interahamwe militia massacred some 800,000 Tutsis and moderate Hutus.

Rwanda twice used the presence of Hutu rebels in eastern Congo to justify invasion, first in 1996 and again in 1998 as part of an intervention that helped spark a five-year war, killing an estimated 4 million, mainly from hunger and disease.

In 2004, two years after the end of the second Congo war, Nkunda led two brigades of soldiers into the bush, saying he needed to protect Tutsis against Rwanda's Hutu FDLR rebels.

Human rights campaigners accuse Nkunda's fighters of abuses including forced displacement, targeted killing of civilians, and recruiting children by force.