Civilians have been fleeing the fighting in the north-east
Rwanda has warned it may send its troops back into the Democratic Republic of Congo.
Rwandan Foreign Minister Charles Muligande said this would happen unless the Ugandan army withdrew from the country, and if the Congolese Government failed to return its forces to positions established in a 1999 ceasefire agreement.
Following last October's peace deal, Rwanda withdrew its forces from DR Congo.
Mr Muligande said the Ugandan and Congolese Governments were deploying troops near areas of eastern Congo that are held by Congolese rebels backed by Rwanda.
Uganda has recently increased its presence near the north-eastern town of Bunia, which it recaptured from the Union of Congolese Patriots rebel group.
Uganda and Rwanda were initially allies in DR Congo but their troops have clashed several times.
Lieutenant Colonel Emmanuel Ndahiro, national security adviser to Rwanda's President Paul Kagame accused
Ugandan government officials and army officers of working with Rwandan rebels based in Congo to destabilize Rwanda.
DR CONGO'S WAR
Seven foreign armies
At least 2 million dead
Disease and abuses widespread
Mr Muligande said this posed an immediate and direct threat to Rwanda's security.
"If the international community has found it acceptable for Uganda to return to the DRC, for all the dubious reasons it has given, then it is legitimate that Rwanda should go into the DRC to contain this new situation," Rwanda's foreign ministry said in a statement.
The fighting in Bunia also threatened the signing of a deal to end the five-year war in DR Congo.
But the Rwanda-backed RCD-Goma group agreed to take part in a power-sharing government and a new unified army.
More than two million people are believed to have died as a result of the war which began in 1998, and at one stage dragged in half a dozen foreign armies.