Congo's government says 250 armed members of a former rebel group have crossed from Uganda into Congo.
Government spokesman Lambert Mende said Sunday the government was surprised by the incursion of two columns of the ex-M23 rebels entered Congo's North Kivu province.
North Kivu governor Julien Paluku warned that the former M23 members carried guns and ammunition and could attack.
M23 operated in eastern Congo from 2012 until it was repulsed by U.N. forces and Congo's army. Many rebels fled to Rwanda and Uganda before a 2013 peace agreement.
Uganda and Congo agreed last year to share intelligence to combat rebel groups active along the countries' border.
Eastern Congo has been plagued by a myriad of armed rebels since the Rwandan genocide in 1994.