The United Nations on Tuesday gave M23 rebel forces 48 hours to leave the city of Goma in the eastern Democratic Republic of Congo or face "the use of force."
A new UN intervention brigade will be used for the first time to help the DR Congo army set up a "security zone" in the city, the UN said.
A statement by the UN mission in DR Congo, MONUSCO, gave M23 rebels in Goma until 4:00pm (2000 GMT) on Thursday "to hand in their weapon to a MONUSCO base" and join a demobilization program.
After then, "they will be considered an imminent threat of physical violence to civilians and MONUSCO will take all necessary measures to disarm them, including by the use of force in accordance with its mandate and rules of engagement."
The UN-proposed security zone includes Goma and its northern suburbs.
The M23 launched a new offensive against the DR Congo army outside Goma on July 14.
"The M23 has used indiscriminate and indirect fire, including by heavy weapons, resulting in civilian casualties," MONUSCO said.
"The M23 has also targeted UN installations with its fire. The security zone will push these indirect fire threats out of range of Goma. The security zone may be expanded and repeated elsewhere, where it is needed."
UN experts and the DR Congo government have said Rwanda has supplied troops and military aid to the M23, allegations denied by Kigali.
The United States last week called on Rwanda to end its backing of the rebel forces.