A group of dissident fighters is closing in on the eastern Democratic Republic of Congo town of Bukavu.
UN peacekeepers have been patrolling Bukavu
The 1,000-strong force has passed the airport and is 30km (20 miles) away.
They are trying to reinforce Col Jules Mutebusi, whose men have been clashing with the army. UN troops have deployed to end the clashes.
The renegade fighters were supposed to be integrated into a new national army and correspondents say these clashes cast doubts on the peace process.
The BBC's Rob Walker in Bukavu says government troops have retreated towards the town, while UN peacekeepers remain in control of the airport, some 35km away.
"There was an hour or two of fighting... between Congolese forces and an advancing
column of about 1,000 men," said UN spokesman Sebastien Lapierre.
He said UN troops had not engaged in the clash.
Within the town itself, the peacekeepers have enforced an ultimatum banning supporters of Col Mutebusi from carrying weapons.
The renegade commander has been suspended from his post in the national army because of the fighting.
Under a peace deal agreed last year, all the Congolese warring factions were supposed to unite as a single army but progress has been slow.
The deal ended five years of war, which dragged in at least six other countries.
A DR Congo vice-president has gone to the area in a bid to negotiate peace between the combatants.
Azarias Ruberwa arrived in Goma, about 120 km (80 miles) north of Bukavu, along with a high-level government delegation.
Mr Ruberwa is a senior figure from the former rebel RCD-Goma - the same group as Col Mutebusi.
Violence between Col Mutebusi's group and regular troops broke out on Wednesday evening.
On Saturday gunmen shot dead one of the UN peacekeepers who have been trying to end the fighting.
The UN accused Col Mutebusi's troops of looting and raping civilians.
Thousands of people have fled into neighbouring Rwanda.
Col Mutebusi and many of his troops are from the Banyamulenge ethnic group - as are most of those who fled.