The UN has withdrawn its backing for a planned offensive against rebels in eastern Democratic Republic of Congo after the government refused to sack two generals, a spokesman has said.
UN troops could not join the offensive because the generals were accused of human rights abuses, he added.
The planned offensive was aimed at disarming the FDLR rebels who are seen as a threat to regional stability.
DR Congo's government has not yet commented on the UN's decision.
It has previously said that it chose its best soldiers to fight the FDLR, and it will not take instructions from the UN.
'Credible evidence'
One of the men chosen to lead the operation, Gen Bruno Mandevu, is on a UN "red list" over 121 alleged human rights violations, including summary executions and rapes, according to Reuters news agency.
The government says he is innocent until proven otherwise.
The FDLR is seen as a threat to neighbouring Rwanda
UN spokesman Nick Birnback told the BBC's Newsday programme that there was "credible evidence" against the generals and "until such time as that has been cleared, we won't work directly with them".
The UN has more than 20,000 troops in DR Congo to help the government restore order in the mostly lawless east.
The nomination of these two red-listed generals has put the 22,000-strong UN mission in DR Congo in a tricky position.
It had been planning the operation against the FDLR for months, and after helping defeat the Tutsi-led M23 rebellion in 2013, it considered that neutralizing the Hutu rebel group was a priority for the stability of the Great Lakes region. Rwanda and the US had been pushing for the operation.
But it can't afford to risk supporting generals who have a record of alleged human rights abuses, especially after the previous operations against the FDLR, in 2009, left 900,000 displaced and 1,000 dead.
While Gen Bruno Mandevu is commander of the anti-FDLR operations, another red-listed General - Fall Sibakwe - has been named commander of a large portion of North Kivu province, where dozens of armed groups operate.
If the Congolese government insists on maintaining these men in their positions, it will make it nearly impossible for the UN mission to carry out its mandate to neutralize armed groups in the region.
This raises the risk that neighbouring Rwanda could get involved more directly in handling the FDLR - it has previously sent its troops into DR Congo to do exactly that.
The FDLR was formed by Rwandans who fled to DR Congo 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.
