The report comes after an upsurge in rebel violence in late August. M23 and the Congolese government have agreed to resume talks, though negotiations have repeatedly stalled.
Vianney Kazarama, a spokesman for the M23 rebels "categorically denied" the report's findings.
"Our leader Sultani Makenga has never trafficked minerals from the time he was in the army until now," Kazarama told The Associated Press.
Eastern Congo's mineral riches have been exploited for years by a myriad of armed rebel groups and militias who have used violence to control the region's mines.
M23 formally launched its rebellion last year, drawing its name from a failed March 23, 2009 peace agreement with the Congolese government. Many of the M23 rebels were previously members of an earlier rebel group that also was involved in gold trafficking.
The Enough Project says Makenga has taken over leadership of gold smuggling operations after former head Bosco Ntaganda surrendered to authorities on war crimes charges at the International Criminal Court. Much of the gold is being moved through a border crossing under M23 control, Lezhnev said.
The M23 rebels are now building alliances with other armed fighters who control the region's mines, the report said. Their partners include militia leader Sheka Ntabo Ntaberi, whose group is accused of raping hundreds of women in Luvungi in 2010.
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Associated Press writer Saleh Mwanamilongo in Kinshasa, Congo contributed to this report.
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