The United Nations on Tuesday threatened to use force against M23 rebel forces near Goma city, eastern Democratic Republic of Congo, if they do not disarm within 48 hours.
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 international body said.
A statement by the UN mission in DR Congo, MONUSCO, gave M23 rebels around Goma until 4:00 pm local time (1400 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, a mainly Tutsi Congolese group founded in 2012, launched a new offensive against the DR Congo army outside Goma on July 14.
Diplomats say fighting in the past two weeks has left hundreds dead.
"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," the statement said.
The new heavily armed 3,000-strong UN intervention brigade in eastern DR Congo is drawn in roughly equal numbers from Malawi, South Africa and Tanzania.
It joins the 17,000 peacekeepers already deployed in the area with MONUSCO, the stabilisation force.
Its mission is to carry out offensive operations, alone or with Congolese troops, against rebel fighters.
Goma is the capital of North Kivu province, which borders two of DR Congo's eastern neighbours, Rwanda and Uganda.
M23 rebels captured the city on November 20 last year, holding it for 10 days. They left only when leaders from the Great Lakes nations of central Africa promised fresh negotiations, opening the Kampala talks.
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 alleged backing of the rebel forces.
Rwanda and DR Congo are both signatories to a UN-brokered peace and security framework signed in March agreeing not to interfere in each other's affairs.
DR Congo further agreed to reform its security forces and take new efforts to spread government authority.
Also Tuesday, the UN refugee agency reported a spike in sexual violence in the region, with 705 cases recorded since January, including 619 cases of rape.
"We are worried that the fighting between the ADF, a Ugandan rebel group, and the (DR Congo) army as well as renewed fighting between the army and the M23 rebels near (Goma) will increase the danger for women in the region, including those living in camps," it said.
More than half of the cases of sexual violence were committed by armed men, the UN High Commissioner for Refugees said in the statement.
On Friday, the government in Kinshasa issued arrest warrants for four of M23's leaders it said had taken refuge in neighbouring Rwanda.
It accused them of "war crimes, crimes against humanity including murder, imprisonment, torture, rape, sexual slavery, ethnic persecution" and several other charges.