"An Ilyushin Il-76 (plane) flew to Libya on February 15 from Baranovichi, a huge former Soviet weapon storage (area) now controlled by the Belarus government," said Hugh Griffiths of the Stockholm International Peace Research Institute (SIPRI), quoting aviation authority sources.
"We strongly suspect it was carrying weapons," he said.
"It was flown to Sebha airport, a very important location for Qaddafi, deep in the desert" in southern Libya," he said, adding "it's one of the very few airports that is still under the control of Qaddafi and that cannot be monitored by naval radar of the US, NATO or European warships, like in Tripoli."
"We've been monitoring (the airport) for a long time because it's been increasingly used to transfer weapons to sub-Saharan countries from Eastern Europe," Griffiths said, also backing up reports at the weekend that a jet owned by Qaddafi had flown to Minsk.
"We've seen flights from Tripoli to Belarus in the last few days," Griffiths said, adding the plane was a Falcon 900, as used by the Qaddafi clan. He noted that Qaddafi's son Khamis especially maintains close relations with Alexander Lukashenko's regime.
SIPRI, which specialises in research on weapons, sent out a warning Monday to relief organisations asking them to be picky about what companies they use to transport humanitarian aid to Libya and Ivory Coast, which according to the United Nations has also been receiving arms from Belarus.
"The humanitarian organisations need big cargo planes and sometimes end up using the same aircraft that transport weapons," Griffiths cautioned.
"Our primary concern has mostly been weapons from Belarus in the last two weeks, weapons that have been flown into Yamoussoukro airport in Ivory Coast and weapons to Sebha airport in Libya," he said.
SIPRI's arms traffic surveillance unit implements European projects under the supervision of Europe's top diplomat Catherine Ashton.