Meanwhile, eight African presidents are set to sign a UN-brokered accord aimed at bringing stability to the region.
As many as 800,000 people have been displaced since the M23 rebel group took up arms against the Kinshasa government last May.
It accuses President Joseph Kabila of failing to honour an earlier peace deal to integrate rebels into the army.
The M23 rebels say they want to improve living conditions for the people of eastern DR Congo, but the UN says they are supported by Rwanda, which has been heavily involved in its eastern neighbour since those responsible for the country's genocide fled there en masse in 1994.
The UN official said the plan for a 2,500-strong intervention force was at an advanced stage, although it has yet to be approved by the Security Council.
The troops would be added to Monusco, the UN's existing DRC peacekeeping mission, mandated to protect civilians.
The new brigades would be tasked with preventing rebels from seizing territory, and weakening them through targeted operations.
Alongside this rapid reaction force, said a UN official, a broad political plan to bring stability to the region is set to be signed on the side-lines of the AU summit in Addis Ababa by the leaders of the DRC, Rwanda, Uganda, Burundi, Angola, the Republic of Congo (Congo-Brazzaville), Tanzania and South Africa.
All of these measures were triggered late last year when M23 rebels seized the eastern city of Goma.
UN peacekeepers chose not to confront the well-armed insurgents, and suffered a blow to their image.
UN Secretary General Ban Ki-moon has been pushing for a comprehensive approach to dealing with the seemingly continuous conflict in the eastern DRC, fuelled by interference from neighbouring states and facilitated by the weakness of the Congolese government and army.
UN experts have cited evidence that Rwanda and Uganda are backing M23, although both countries strongly deny the charges.
"Following the Goma crisis there was a realization here that after more than 10 years of UN engagement in the DRC there was a need to look at things differently to break the cycle of violence in the east," said the UN official.
"So we tried to go for the heart of the problem and see why these cycles are on-going."
The framework agreement involves a commitment from Kinshasa to strengthen the Congolese army and pledges of non-interference from its neighbours.
It also seeks to address the legitimate concerns of all countries concerned, says the UN official, "meaning there are legitimate concerns for all countries involved, including Rwanda and Uganda".
The UN intervention force would not be a substitute for the Congolese army, said the peacekeeping official. Rather it would "create a specific space and time for the political process, and for the army to build at least some capacity."
The idea would be to recruit African soldiers. Only Tanzania has offered troops so far.
The combat brigades would be aided by surveillance drones recently authorised by the Security Council, which are also meant to monitor the borders.
The Secretary General will present detailed plans for the "peace-making force" at the beginning of February and, pending Security Council approval, the aim is to get troops on the ground within three months.
The UN wants to set up an intervention force to fight rebels fuelling conflict in DR Congo, says a UN official.