Last month, the rebels came within several dozen kilometres of Goma, a commercial hub of over one million people, on the border with Rwanda.
Their advance has cut off the RN2 highway, which leads to Goma from the north, and with it access to farm produce from North Kivu's Rutshuru region.
Boats can also reach Goma from Bukavu, a city on the other side of Lake Kivu. And goods can still enter from Rwanda.
But inside the DRC, only the road leading to the city from the west remains viable, slashing trade volumes and driving up prices.
Some fear M23 rebels plan to block this artery too in order to exert pressure on the government.
Onesphore Sematumba, a DRC analyst for the International Crisis Group (ICG), said the rebels aim to sweep into Masisi territory -- west of Goma -- to this end.
"If we can't negotiate a humanitarian corridor for the city, it will be a catastrophe," he said.
Prices for basic items such as soap surged 20 percent in some Goma markets between October and November, according to a food security group led by the UN's Food and Agriculture Organization (FAO) and World Food Program (WFP).
Several people interviewed by AFP also said the cost of a cup of beans had tripled from the equivalent of about $0.5 to $1.5.
Claudia Ngowa, 36, said she had had to shutter her small soap factory and furlough four employees because she imported raw materials from Uganda along the RN2.
The entrepreneur also owns a farm in Masisi, but said rampant insecurity makes it difficult to transport goods despite the road remaining untouched by the M23.
"We can't go on without selling," she said. "It's really precarious for everyone."