The UN's peacekeeping mission in Mali has acknowledged there were "dysfunctions" that preceded a row with the Malian authorities sparked by the arrival of Ivorian troops at Bamako airport this month.
Mali's military-led government says 49 Ivorian soldiers were detained after landing on a special flight on July 10 without supporting documents and has described them as "mercenaries".
But Ivory Coast says they were sent to provide routine back-up duties for the Ivorian contingent in the MINUSMA peacekeeping operation, and is demanding their release.
Asked by the government to explain the situation, MINUSMA said in a reply seen by AFP "it appears that certain measures were not taken."
"The Mission is trying to understand how these dysfunctions were able to occur in order to avoid them occurring again in the future," it said.
The message to the Malian foreign ministry, known as a note verbale, was confirmed by the Malian authorities on Monday as being genuine. There was no immediate response from MINUSMA to a request for comment.
The spat takes place against a backdrop of problems in Mali, one of Africa's poorest and most unstable countries.
Mali is struggling with a long-running jihadist insurgency that has claimed thousands of lives and forced hundreds of thousands from their homes.
It is also in the grip of political turbulence after colonels angry at the government's handling of the insurgency seized power in August 2020.
The MINUSMA message pointed to an apparent discrepancy in the records about the Ivorians' contracts as "national support elements" (NSEs) -- external personnel hired to provide support duties.
"The Ivorians were deployed at Senou (in Bamako) to provide security for German NSEs there, instead of at Timbuktu (in northern Mali) where the Ivorian contingent of MINUSMA is based," it said.
"MINUSMA is not aware of a contract between Germany and third parties for the protection of the German NSE base."
MINUSMA added that it had not been informed of the presence and number of Ivorian NSEs in Mali.
"However, by correlating internal documents, MINUSMA estimates that the Ivorian detachment amounts to 50 men and women."
The row has strained Mali's relations with Ivory Coast, a leading regional power and member of the West African bloc ECOWAS, which has been at odds with the junta.
It has also cast a shadow over Mali's relations with MINUSMA.
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The United Nations Multidimensional Integrated Stabilization Mission in Mali has been in the country since 2013. Its 17,600 troops, police, civilians and volunteers make it one of the UN's biggest peacekeeping missions.
On Wednesday, the Malian authorities ordered the expulsion of MINUSMA's spokesman, Olivier Salgado, accusing him of making "unacceptable" postings on Twitter that supported Ivory Coast's version of events.
Togo is acting as a mediator between Ivory Coast and Mali.