A leading rights group in the Democratic Republic of Congo has called on the government to annul a controversial general's appointment to lead the fight against rebels.
Gen Bruno Mandevu could not run operations against the FDLR rebel group because he was accused of summary executions and rape, it said.
The government said he remained innocent, until proven otherwise.
It is under international pressure to disarm the FDLR in eastern DR Congo.
The FDLR was formed by Rwandans who fled to the mostly lawless east after the 1994 genocide in Rwanda.
Rwanda sees the FDLR as a threat to its stability and, along with the UN, has repeatedly demanded its disarmament.
It has twice invaded DR Congo, saying it is trying to hunt down the militiamen who took part in the genocide.
Rwanda's critics say many FDLR fighters are too young to have taken part in the genocide.
The FDLR has been accused of recruiting child soldiers, rape and systematic looting.
A 2 January deadline to launch the operation passed without any action.
On Sunday, DR Congo's government announced Gen Mandevu's appointment to head the offensive.
UN forces in DR Congo, known by the acronym Monusco, were expected to take part in the assault.
Reuters news agency quoted unnamed Western diplomats as saying this was in doubt following Gen Mandevu's appointment as he was on a Monusco "red list" over 121 rights violations, including summary executions and rapes.
He has not commented on the allegations.
'Innocent until proven guilty'
The Congolese Association for Access to Justice campaign group said he should be removed from the post, pending an independent investigation into the allegations.
"We ask the United Nations to refrain from collaborating with General Mandevu", said Georges Kapiamba, the head of the group.
The head of the UN mission in DR Congo, Martin Kobler, told the BBC that the UN was in contact with the government over Gen Mandevu's appointment.
However, it whole-heartedly supported an assault on the FDLR, and believed that any risk of human rights violations could be "mitigated" by close surveillance of the operation, he said.
DR Congo government spokesman Lambert Mende said the government would not take instructions from the UN.
Gen Mandevu was innocent until proven guilty, and the government was waiting for the UN to present evidence against him, he said.
The government had chosen its best soldiers to expel the FLDR from its territory and this included the general, Mr Mende added.