Kinshasa said Tuesday Rwanda's accession to the UN Security Council as a non-permanent member amounted to condoning violence it blames on Kigali in the eastern DR Congo provinces.
Together with Luxembourg, Argentina, South Korea and Australia, Rwanda was elected to the 15-member Security Council last week.
"This decision is a very bad signal because it trivialises the horrific situation created by this country in the Democratic Republic of Congo, where it continues to disrupt peace and security," government spokesman Lambert Mende said.
A UN report has accused Rwanda of being behind a rebellion in eastern DR Congo called the Congolese Revolutionary Army (ARC), formerly known as the M23 movement.
Rights groups have accused the ARC-M23 -- which has repeatedly threatened to attack the main eastern city of Goma -- of war crimes, including summary executions, rape and forced recruitment.
One of the rebels' main leaders is Bosco Ntaganda, a rogue Rwandan-born general nicknamed "The Terminator" and wanted by the International Criminal Court for crimes against humanity.
Kigali has denied the accusations and in turn accused Kinshasa of planning attacks against the Rwanda regime in cooperation with a group of Hutu rebels believed to include 1994 genocide suspects.
The M23 emerged earlier this year when former rebels who had been integrated into the regular Congolese army in 2009 mutinied, ostensibly over pay.
Regional analysts however say the new rebellion was over access to the region's vast mineral wealth, which Rwanda has long been accused of covertly exploiting via local proxies.
Congolese President Joseph Kabila on Tuesday held talks with his South African counterpart Jacob Zuma in Pretoria.
In a joint statement, they "condemned in the strongest possible terms those forces that are involved in destabilising a sovereign state and called on them to cease their activities immediately."
South Africa, which also has huge stakes in DR Congo's mining industry, is working with the Southern African Development Community to resolve the spiralling crisis between the two neighbours.
Officials in Goma announced on Tuesday that two key border crossings between DR Congo and Rwanda would from now on remain closed at night, from 6:00 pm (1600 GMT) to 6:00 am (0400 GMT).
Several thousand people cross the border every day and the local Congolese governor said in a statement that the decision stemmed "from a surge in criminal activity along the border."