Thousands of people fled the area of clashes around the town of Sake, as M23 rebels rushed from Goma to reinforce their positions there against a counter-offensive by the army.
The rebels, thought to be Rwandan-backed, have vowed to "liberate" the resource-rich country after taking Goma, a provincial capital near Rwanda.
The head of M23's political arm said the rebels would not retreat despite the call to do so from governments in central Africa, preferring to hold their ground until President Joseph Kabila opens direct talks with them. "We'll stay in Goma waiting for negotiations," Jean-Marie Runiga said. "They're going to attack us and we're going to defend ourselves and keep on advancing."
Rebel fighters seized the sprawling lakeside city of a million people on Tuesday after government soldiers retreated and UN peacekeepers gave up trying to defend it.
The next day the rebels moved in unopposed to Sake, a strategic town about 15 miles west along the main road. It was there that government troops and allied militia were hitting back in fighting the flared up late yesterday.