The Democratic Republic of Congo's government is deploying 10,000 troops in the east of the country, to confront dissident forces.
The government is said to be deploying 10,000 troops to the east
Observers fear renewed fighting could shatter DR Congo's fragile peace, following five years of war.
Rwanda, which backed rebels during the war, has responded by accusing DR Congo of preparing to attack.
The Congolese defence minister has said that his country has no intention of attacking its neighbour.
Jean-Pierre Ondekane told reporters in the capital, Kinshasa, that the deployment of troops to the east of the country was intended to help the integration of former rivals into a national army.
The South African President, Thabo Mbeki, has described the possibility of war between Congo and Rwanda as potentially catastrophic.
An estimated three million people in Congo were killed during more than five years of war which sucked in neighbouring countries.
United Nations peacekeepers have fired on rebel troops from a helicopter, a UN spokesman says.
There are no details on casualties or the circumstances of the clash, which occurred near the eastern town of Kamanyola, said the UN's Sebastien Lapierre.
The DR Congo troops are being sent to the towns of Beni, Kindu and Kalemie and near the Rwandan border, diplomats say.
"We shall not sit back and watch these developments, we have a country and people to defend," Rwandan Foreign Minister Charles Muligande told the BBC.
Tension is high after dissident troops captured the Congolese border town of Bukavu earlier this month.
President Joseph Kabila sacked three of his military leaders over the weekend, in the wake of the Bukavu defeat, which was followed by an alleged coup attempt.
The armed forces chief of staff, the head of the presidential guard and the head of the "maison militaire", the president's team of military advisers were suspended for "reasons of efficiency and to accelerate the integration process".
Under a peace deal signed last year, former rebel groups are being integrated into a new, unified national army but progress has been slow.
And tensions between the former enemies were one factor in the Bukavu fighting.
Mr Muligande told the BBC his country was concerned at what he claimed was the deployment of troops in offensive positions in key towns in the east of DR Congo.
A Congolese military spokesman did not give details of the deployment but said that the army was "doing its job" following the recent unrest.
"After the problems we have had, the president called for a general mobilisation. This is what we are doing," Colonel Leon Kasonga said.
BBC regional analyst Martin Plaut says the current situation is explosive.
The former rebel RCD, based in the east, says it has 40,000 troops - and has warned that any government deployment could lead to serious clashes.
The RCD is part of a power-sharing government set up in negotiations to end the war.
More than 30,000 people have fled the recent fighting, say officials
African leaders and the United Nations are making efforts to contain the situation to stop another regional escalation.
Renegade General Laurent Nkunda, who briefly captured Bukavu, is an RCD officer, based in their headquarters, Goma.
Officials say more than 1,000 refugees a day are arriving in Burundi, amid fears of renewed fighting.
Most of the early arrivals were from the same Banyamulenge ethnic group as the rebels, who fear reprisals from government soldiers.
"I do not know who was shooting or who has control of the area, but there was a heavy exchange of gunfire," said refugee Maze Kahindo, who arrived in Burundi on Saturday.
Gen Nkunda had said that his fellow ethnic Banyamulenges were being targeted and killed by the army, but the UN dismissed his claims that he was preventing a genocide.
The Banyamulenge are ethnic Tutsis, who have lived in DR Congo for several generations but who retain ties to Rwanda.