A group of about 100 Rwandan troops has been spotted inside the Democratic Republic of Congo in a first sighting by United Nations observers.
The Congolese are to send thousands more reinforcements to the east
Thousands of civilians have been fleeing renewed fighting in the east.
The Congolese say more than 6,000 Rwandans have crossed the border and are attacking and burning villages.
Rwanda's president had threatened to send troops across the border to engage Hutu rebels inside Congolese territory who have not been disarmed.
A UN spokesperson said a team of peacekeepers had seen about hundred soldiers, who they thought were Rwandan, near the border town of Goma.
"Infiltration is nothing new but this is something else, it has the appearance of an invasion," Monuc's chief in Goma, M'Hand Djalouzi, told journalists.
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 warned that 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.
But some Congolese analysts say that the real reason behind Rwanda's threats is that President Joseph Kabila has recalled the governor of North Kivu province, based in Goma, who is from the Rwandan-backed RCD former rebel group.
They say Rwanda wants to ensure it retains control of the border area.
On Wednesday, from the eastern town of Beni, Congolese regional cooperation minister Mbusa Nyamwisi said villages were being targeted nearby.
"We are being attacked by the Rwandan troops," he said
Earlier, President Paul Kagame said in a speech that Rwandan troops may already have crossed into DR Congo in pursuit of ethnic Hutu rebels.
He told senators attempts to disarm forces across the border "will not take long, or it is even happening now".
Rwandan troops were first reported by diplomats to be in the DR Congo on Monday.
The DR Congo authorities say they will send more than 6,000 troops to the border area within the next two weeks.
Residents of the Congolese border town of Bukavu have reportedly been gathering stones to use to fight off any Rwandan incursion.
Rwanda has twice invaded its much larger neighbour - in 1996 and 1998 - accusing successive Congolese governments of backing the Hutu rebels.
DR Congo's majestic mountains are ideal rebel hide-outs
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.
Under that deal, the Hutu rebels were supposed to have been disarmed but progress has been slow.
Rwanda says that 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.