The conflict in DR Congo is said to stem from Rwanda's 1994 genocide
Congolese soldiers have been accused of barring UN troops and aid staff from an area where an operation with Rwandan troops is ongoing on a Hutu militia.
A UN spokesman in the Democratic Republic of Congo told the BBC it was "deplorable", when so many displaced people were in desperate need of help.
At least 3,000 Rwandan troops have crossed the border for the joint push on Rwandan FDLR fighters since Tuesday.
The rebel group's leaders have been linked to the 1994 Rwandan genocide.
DR Congo and Rwanda agreed last month to take joint action against the Hutu rebels of the Democratic Forces for the Liberation of Rwanda (FDLR), which is estimated to be more than 6,000-strong.
The BBC's Thomas Fessy in the Congolese capital, Kinshasa, says hundreds more Rwandan troops entered North Kivu province on Wednesday, according to UN sources, following the 2,000 who crossed over the day before.
RWANDA IN DR CONGO
1994: After Rwanda genocide, Hutu rebels move to DR Congo (then Zaire)
1997: Rwanda invades DR Congo to deter cross-border rebel raids
1998: Rwanda invades again. At least four other nations intervene
2002: Rwandan military leaves DR Congo
2003: "Africa's world war" declared over
2008: Congolese Tutsi rebel offensive on Hutus
2009: Rwandan military enters DR Congo
Our correspondent says there has been no fighting reported so far as both armies appear to be still making preparations.
Some troops are heading towards Rutshuru, 70km (45 miles) north of Goma, and others have been deployed near the strategic town of Sake, 30km (12 miles) west of the provincial capital.
The UN Mission in the Democratic Republic of Congo (Monuc) said its peacekeepers and aid workers had been blocked by Congolese troops at checkpoints north of Goma.
Monuc military spokesman Lt Col Jean Paul Dietrich told the BBC's Focus on Africa programme they had protested to the Congolese authorities.
"We've just tried to negotiate since yesterday that our force should have the [access] guarantee," he said.
"Not only for us, the Monuc force, but as well all humanitarian [agencies] desperately need to get to those IDPs [internally displaced persons] in those areas.
"And so far it seems that the blockade cannot be lifted and we deplore that and we cannot accept that. We will continue to keep up pressure."
AFP news agency reported that a convoy of Indian UN peacekeepers and a Red Cross vehicle had been turned away.
The Congolese authorities have said the operation to flush out the FLDR would last between 10 and 15 days, but Lt Col Dietrich said this would be a "very challenging task" in such a short timeframe.
Congolese President Joseph Kabila has reportedly come under fire for not informing parliament about the invitation to Rwandan troops.
"If what I'm told is true, it's quite simply grave. It raises lots of questions," parliamentary speaker Vital Kamerhe told the UN-run Radio Okapi.
The UN and aid agencies have raised concerns about the threat posed to civilians on the ground.
Some 250,000 people have been displaced since last August alone
Rwanda twice invaded its much larger neighbour during the 1990s, saying it was pursuing the FDLR.
But analysts say much of the fighting is also linked to eastern DR Congo's rich mineral resources, which all sides have been accused of plundering.
Action against the Hutu fighters has been a key demand of the Congolese Tutsi rebel CNDP (National Congress for the Defence of the People), which declared a ceasefire last week.
The CNDP, which seized swathes of territory last year, has offered to join Congolese government troops in fighting the FDLR.
Our correspondent says the CNDP may well be part of the operation, but no details have been given about how they could be integrated to the Congolese army.
On-and-off fighting involving the CNDP, FDLR, the army and pro-government militias has forced more than one million people in North Kivu to flee their homes since late 2006.
Before last month's deal was signed, the UN accused Rwanda and DR Congo of fighting a proxy war in the region - with Rwanda backing Gen Nkunda and DR Congo of working with the FDLR.