File Photo: MONUSCO troops in northeastern DRC last December. AFP
The MONUSCO mission condemned "the hostile acts of M23" rebel group and called for an immediate halt to the fighting.
It was providing "air support, intelligence and equipment" as well as medical assistance to the DRC's army, the mission said on Twitter.
The peacekeepers said they were "mobilised in support" of the FARDC army which clashed this week with the M23 around a strategic highway, with residents reporting at least ten people dead since Sunday and dozens more injured.
MONUSCO said it had set up an "operations coordination centre" with the army and was carrying out reconnaissance and surveillance flights, but did not provide further details about the alert level.
Several witnesses told AFP by telephone on Saturday that the M23 had seized control of Kiwanja and Rutshuru-centre, along the RN2 highway linking the North Kivu provincial capital Goma with the north and Uganda.
M23, a mostly Congolese Tutsi group, resumed fighting in late 2021 after lying dormant for years.
It has since captured swathes of territory in North Kivu province, including the strategic town of Bunagana on the Ugandan border in June.
The group's resurgence has destabilised regional relations in central Africa, with the DRC accusing its smaller neighbour Rwanda of backing the militia.
The rebel group first leapt to prominence in 2012 when it briefly captured Goma before a joint Congolese-UN offensive drove it out.
The militia is one of scores of armed groups that roam eastern DRC, many of them a legacy of two regional wars that flared late last century.