The United Nations has given the rebel forces 48 hours to leave the eastern Democratic Republic of Congo city of Goma or face "the use of force", according to AFP news agency.
A new intervention brigade will be used for the first time to help the DR Congo army set up a "security zone" in the city, which has come under renewed threat from the 23 rebel forces in recent weeks, the UN said on Tuesday.
[After deadline] they will be considered an imminent threat of physical violence to civilians and MONUSCO
A statement by the UN mission in DR Congo, MONUSCO, gave rebels in Goma until 4:00pm local time (20:00 GMT) on Thursday "to hand in their weapon to a MONUSCO base" and join a demobilisation programme.
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 offencive 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 rebels, allegations denied by Kigali.
The United States last week called on Rwanda to end its backing of the rebel forces.