Kivu Security Tracker (KST), a respected violence monitor, said on Wednesday that the M23 was advancing in North Kivu's Masisi territory.
The Tutsi-led group had cut off the road leading between the town of Kitchanga -- which is in Masisi -- and Goma, it said.
Police and local government officials in the area confirmed to AFP on Thursday that the road had been cut off.
The artery is considered vital for supplying Goma, especially since the main highway leading north out of the city has been cut off since an earlier M23 advance.
A resident of the village of Kilolirwe, in Masisi, said that clashes with the rebels were also ongoing near Kitchanga.
An army official, who declined to be named, also said that clashes there were continuing.
Heritier Ndangendange, the spokesman for the APCLS militia, described the situation in Kitchanga as "tense" but said the APCLS and allied fighters remained in control there.
The fresh clashes come after the M23 was required to withdraw from territory it occupies, under the terms of regional mediation efforts.
Former Kenyan president Uhuru Kenyatta, a mediator for the East African Community (EAC) bloc, warned on Wednesday the situation was "sharply deteriorating."
The fighting has triggered a humanitarian crisis and led to a row between DR Congo and neighboring Rwanda.
The government in Kinshasa accuses Rwanda of supporting the M23 -- something that Kigali denies.
The M23 is among scores of armed groups who roam the east of the mineral-rich Democratic Republic of Congo.
The M23 has conquered swathes of territory in North Kivu province since last year and advanced towards its capital Goma.
The group first leapt to prominence in 2012 when it captured the city of more than one million people, before being driven out and going to ground the following year.