United Nations peacekeepers are back in control of the town of Sake in eastern Democratic Republic of Congo after heavy clashes with dissidents.
Mr Nkunda says the minority Tutsi community has been excluded
A UN spokesman told the BBC that the UN sent in troops and attack helicopters and pushed back soldiers loyal to dissident general Laurent Nkunda.
A rebel advance on the regional capital, Goma, was also halted. There were no casualties among UN troops.
The clashes are the most serious in the east since elections earlier this year.
Thousands of civilians fled their homes in the town. Casualties are unclear.
Tension is already high in the capital, Kinshasa, where the Supreme Court is due to deliver its verdict on the legal challenge to last month's presidential run-off election brought by the former rebel leader and current Vice-President Jean-Pierre Bemba.
In his appeal, Mr Bemba said the results that gave victory to President Joseph Kabila did not reflect the reality at the ballot box.
United Nations forces said their helicopters and ground troops opened fire against the dissidents after being attacked in and around the volatile eastern town of Sake - which is permissible under their mandate.
"Our troops were fired upon and they returned the fire," said Monuc spokesman Kemal Saiki told the BBC's Focus on Africa.
"Our forces are controlling Sake and the Congolese armed forces have been able to regain the hills around Sake. The troops that tried to move to Goma have been stopped in their tracks but there is still occasional sporadic firing," he said
At least eight soldiers are reported to have been killed in the attacks by the dissidents on Sake and eight rebels killed.
The dissident troops used machine-guns, mortars and rocket-propelled grenades.
Sake is a small town about 25km (15 miles) west of the provincial capital of Goma.
Mr Nkunda left the army and launched his own low-level rebellion after DR Congo's war ended, saying that the country's transition to democracy was flawed and had excluded the minority Tutsi community.