Some 10,000 troops are being sent to expel Rwandan forces from the Democratic Republic of Congo, President Joseph Kabila has said.
The Congolese troops are ill-equipped to halt a major invasion
He accused Rwanda of invading to loot DR Congo's natural resources.
Rwanda has denied that its forces have entered DR Congo. The UN says there were indications, but "no proof", of Rwanda's presence.
Rwanda has twice invaded DR Congo in recent years - it says to attack Rwandan rebels based there.
"Rwanda has goals that are political, economic, exploitative and predatory," Mr Kabila said, in his first public reaction since reports emerged that Rwanda had sent troops into eastern DR Congo.
Rwandan President Paul Kagame has said that military action against ethnic Hutu rebels was "imminent" but promised not to attack Congolese troops.
But the BBC's Mark Doyle, who has just returned from the region, says that any Rwandan military action could unravel tentative moves towards peace throughout central Africa.
Following a debate on the crisis at the UN Security Council in New York, Council President Abdallah Baali from Algeria said: "The general sense is that there were Rwandan troops,
although nobody can really confirm it in the clearest way."
Mamadou Bah, a spokesman for the UN mission in DR Congo (Monuc), said: "Our helicopter reconnaissance patrols have been able to take photos of abandoned bivouacs and well-equipped soldiers who are moving with new uniforms and materials."
But Mr Kagame's adviser on DR Congo, Richard Sezibera, insisted that there were no Rwandan troops across the border.
"All reported sightings of Rwandan troops in the DRC are false. Rwanda does not have any troops in the Democratic Republic of Congo," he said.
The Congolese government said 6,000 Rwandan troops had crossed the border and attacked villages.
Some 2,000 people have fled amid reports of the Rwandan advance in North Kivu province, says the UN.
The Security Council urged Rwanda not to send troops into DR Congo but did not condemn Rwanda's action or impose sanctions on Mr Kagame, as the Congolese had wanted.
Nigeria's President Olusegun Obasanjo, chairman of the African Union, has told the BBC that Rwanda has a case in its conflict with DR Congo but that his authority would be undermined if Rwandan troops had entered DR Congo.
Rwanda has consistently said it is prepared to take military action because of the threat it says is posed by the group of some 8,000 men, which includes fighters who took part in the 1994 genocide of Tutsis and moderate Hutus.
Rwanda 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".
These armies were all accused by the UN of exploiting DR Congo's rich natural resources, including gold and diamonds.
Under the peace deal, the Hutu rebels were supposed to have been disarmed but progress has been slow.
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.