UN and Democratic Republic of Congo troops are reinforcing a key city in the east of the country to guard against attack by rebels who have seized ground in recent days.
DR Congo authorities and the United Nations fear that the M23 movement, which took one town on the Uganda border last week and forced 600 government troops to flee, may target the provincial capital of Goma, UN officials said.
"It would be disastrous if Goma was taken," said a UN official who gave details of the reinforcements on Tuesday.
The UN Security Council is to discuss the new strife on Tuesday while international leaders will use an African Union summit in Addis Ababa this week to try to defuse tensions between DR Congo and Rwanda over the fighting.
M23, a group of mutineers led by accused war criminal Bosco Ntaganda, has already briefly taken other towns near its new stronghold in Bunagana.
The DR Congo government is moving a US-trained battalion from the north of the country to eastern Goma, the official told reporters, speaking on condition of anonymity.
The battalion, previously used in the hunt for Lord's Resistance Army (LRA) fighters, will join about 7,000 troops already in Nord Kivu province, of which Goma is the capital.
The UN mission in DR Congo, known by the acronym MONUSCO, is moving Ghanaian troops and Guatemalan, Jordanian and Egyptian special forces from its 18,000-strong peacekeeping force to the city, said the UN official.
MONUSCO's deputy forces commander, General Adrian Foster of Britain, has moved to Goma to run the UN operation, as UN troops will help with planning, logistics, fuel, transport and other support.
They have already gone into battle to protect civilians, and one Indian peacekeeper was killed last Friday.
"This is all to ensure that we can strengthen our support to ensure that Goma does not fall and also to provide wider protection of civilians in the area affected by the M23," said the UN official.
"M23 rebels told us they're moving troops south, and that's where the Congo troops are preparing their defences," Al Jazeera's Peter Greste said.
"This is traditional stronghold for Congloese Tutsis, so fighters from M23 know the area well."
M23 broke away from the government army in April complaining about conditions. In the past two weeks its numbers have grown from about 1,000 to 2,000 fighters.
At the weekend, M23 took Rutshuru, which controls a key highway to Goma, and other smaller nearby towns. But they withdrew again on Monday without any apparent reason.
"Nobody knows what the intentions of the M23 are right now," said the UN official. "Some have been seen going back along the road to Bunagana. Others are up fairly close to the small towns that they had taken.
"Our great concern is that the M23, having taken these towns, would then be planning some sort of advance against Goma."
The army is poorly trained and equipped. Government troops "withdrew from a number of their former positions as the M23 advanced", the UN official said.
MONUSCO has told the UN headquarters that "the M23 forces appear well equipped, well supplied, they have recently got more troops," said the official.
The DR Congo government has accused neighboring Rwanda of supporting M23.
Rwanda has strongly denied the claims, though a recent report by a panel of UN sanctions experts said fighters and weapons used by M23 have come from Rwanda.
The UN is also concerned that M23 is trying to form alliances with other rebel groups in the region.
Alongside the reinforcements, the UN will seek to press diplomatic means to defuse the crisis at the AU summit this week. The International Conference of the Great Lakes Region - which includes DR Congo and Rwanda - is to be held on Thursday focusing on the new troubles.
Security Council powers are also trying to get countries with influence over DR Congo and Rwanda to bring them back from the brink of a showdown, diplomats said ahead of Tuesday's council meeting.