That is not exactly true Cyclic Hotline. According to ICAO rules, the State of Occurrence is responsible for the investigation of the accident, if they are not capable (to small, no resources etc) then the State of Registration, and/or the State of Manufacture can lead the investigation. In this instance, Sudan have asked the UN for help, of which, ICAO is part. As for all UN aircraft accident reports being kept secret, well, that is not an entirely accurate statement either. I happen to know a gentleman who investigated two UN C-130's shot down in Angola. At that point in time he ws working for a Civil Aviation Authority of a country that was considered neutral by the 2 warring factions in Angola. He was then attached to the UN through ICAO to participate in the investigation.
After a quick, and by no means comprehensive, search through the net, I found reports for the following accidents:
date type reg operator loc
19-MAR-2000 Antonov 26 UR-26586 UN D.R. Congo
12-NOV-1999 ATR-42 F-OHFV UN Yugoslavia
02-JAN-1999 Lockheed L-100 D2-EHD UN Angola
26-DEC-1998 Lockheed L-100 S9-CAO UN Angola
22-NOV-1995 Antonov 12 RA-11008 UN Angola
31-JUL-1994 Antonov 26 UR-26207 UN Croatia
09-FEB-1993 Antonov 32 ? UN Kenya
09-AUG-1974 DHC-5 Buffalo 115461 UN Syria
06-MAR-1966 Curtiss C-46 SE-CFG UN D.R. Congo
20-SEP-1962 Douglas DC-3 UN-202 UN D.R. Congo
I'm confident a more thorough search would bring up more results. The problem with finidng UN accidents can sometimes be that the aircraft are contracted through a third party, and those details are the ones on the accident report.
I hope this clears up some confusion that seems to be surrounding this topic.
noooby