War fears after Rwanda 'invasion' - first of 5,000 extra UN peacekeepers arrived in DR Congo
The UN Security Council is set to hold an emergency session to discuss the reported incursion of Rwandan troops into eastern DR Congo.
Rwanda's president has assured the African Union that Rwandan military action in DR Congo will target ethnic Hutu rebels and not Congolese forces.
But there are fears growing insecurity could threaten peace across the region.
Ugandan troops are reported to have deployed troops along their border with DR Congo as a precautionary measure.
UN peacekeepers say they have seen about 100 soldiers they believe are Rwandan.
The Congolese government said 6,000 Rwandan troops had crossed the border and attacked villages.
It has asked the Security Council to condemn Rwanda's action and impose sanctions against Rwanda's President, Paul Kagame.
In a letter to Nigeria's leader, who chairs the African Union, Mr Kagame said he expected his troops to finish their mission in two weeks.
Mr Kagame has not said whether the operation had started.
He sent the letter last week, but its contents have only just been revealed.
The United States and European Union have urged Rwanda and DR Congo to solve their dispute peacefully.
A senior US diplomat, Donald Yamamoto, is travelling to the region in the next few days in an effort to persuade the two sides to solve the crisis peacefully.
Thousands of civilians have been fleeing renewed fighting in the north-east, according to a UN humanitarian agency.
Last week, the UN warned Rwanda not to use military force, saying such a move could undermine international efforts to stabilise the region.
Rwanda has consistently said it is prepared to take military action because of the threat it says is posed by the group which include fighters who took part in the 1994 genocide of Tutsis and moderate Hutus.
The Congolese government says Rwanda's action has little to do with any rebel threat but is part of their efforts to dominate and exploit eastern DR Congo economically - as the region is full of valuable minerals such as gold and diamonds.
But the BBC's Mark Doyle, who has just returned from the region, says the wider significance of any Rwandan military action is that it could unravel tentative moves towards peace throughout central Africa.
The DR Congo authorities say they will send more than 6,000 troops to the border area within the next two weeks.
Rwanda has twice invaded its much larger neighbour - in 1996 and 1998 - accusing successive Congolese governments of backing the Hutu rebels.
It withdrew its troops in 2002, under a regional deal to end five years of war in DR Congo, in which some three million people died.
The armies of at least six foreign nations - and countless rebel groups - were embroiled in "Africa's first world war".
Under that deal, the Hutu rebels were supposed to have been disarmed but progress has been slow.
Rwanda says the rebels are now attacking its territory under the noses of the international community.
Last week, the first of 5,000 extra UN peacekeepers arrived in DR Congo.
There are already more than 10,000 UN peacekeepers in DR Congo; troops have been placed on alert and patrols have been despatched to check for any Rwandan incursion.