The United Nations is investigating claims that a general set to head its force in Sudan's Darfur region, participated in the Rwandan genocide.
African Union soldiers in Darfur are to be joined by new troops
UN spokesman Yves Sorokobi said human rights groups should submit evidence inking Rwandan General Karenzi Karake to any alleged crimes.
The African Union approved General Karake to become the deputy commander of the AU-UN hybrid force in Darfur.
Some 800,000 Tutsis and moderate Hutus died in the 1994 genocide in Rwanda.
A Belgium-based Rwandan exile group has accused General Karake of supervising the killings of civilians during the genocide in Rwanda and the DR Congo.
"We are taking the allegations very seriously and we have invited the groups to forward them so that we can do an independent background check," Mr Sorokobi told the BBC's Network Africa programme.
7,000 - existing AU force
1,000 - pledged by Senegal
800 - pledged by Malawi
Nigeria, Burkina Faso, Cameroon, Ethiopia and Egypt
Indonesia, Pakistan, Nepal and Bangladesh
26,000 - total planned
Rwanda's Foreign Ministry has dismissed the claims as a mere fabrication and an attempt to tarnish Rwanda's image.
"Major-General Karake is a well-trained and experienced senior officer who has ably served in various senior command staff roles in the Rwanda Defence Forces and rightly deserves the post," the statement said.
The UN Security council has approved a 26,000 strong joint AU-UN peacekeeping force to protect more than two million civilians displaced by the fighting in Darfur region.
Rwanda sent 2,000 of the 7,000 AU troops currently in Darfur.
At least 200,000 people are estimated to have died and more than two million have fled their homes in Darfur since fighting broke out in 2003.
Sudan's Arab dominated government, and the pro-government Janjaweed militias, are accused of war crimes against the region's black African population - although the UN has stopped short of calling it genocide.
Sudan has always denied backing the Janjaweed militias and argued that the problems in Darfur were being exaggerated for political reasons.
Meanwhile the UN has asked the Sudanese government to investigate allegations of rape and sexual slavery committed during raids on nine villages in eastern Darfur last December.
A report by the UN Human rights commission says 50 women were abducted and raped by members of the Sudanese Army and allied militia in Jebel Marra region.
"The women were subjected to multiple rape and other forms of violence which constitute war crimes," the report from the office of Louise Arbour said.
The Sudanese government had pledged to investigate the allegations last April when the report first emerged but no action has been taken.
During the attack 36 people were killed at Deribat, one of the nine villages raided by the soldiers.
The UN says the force in question was loyal to a commander who was appointed governor of West Darfur in February this year.
The accusations could increase Sudanese government fears that the planned UN peacekeeping force could arrest those allegedly involved, and take them to the International Criminal Court in the Hague.