Belarus' Foreign Ministry said in a press release that it had summoned Lithuania's ambassador, Andrius Pulokas, to hand him "the diplomatic note expressing concern of Belarus that it was not promptly notified about the incident at the site of the Ignalina NPP".
The ministry called on Lithuania "to conduct a thorough investigation of the incident and inform Belarus of its results".
The 15min.lt news website reported on Monday that the incident had occurred during the so-called hot tests in late 2017.
According to the report, it turned out while moving radioactive waste out of old storage facilities into new ones that the radioactivity level of the waste was higher than estimated.
The Lithuanian State Nuclear Power Safety Inspectorate (VATESI) said on Tuesday that the event was not a nuclear incident under the International Nuclear and Radiological Event Scale (INES).
Lithuania accuses Belarus of failing to comply with international safety standards in building its new nuclear power plant in Astravyets, some 40 kilometers from Vilnius.