The operator shall report any accident or incident involving DG to the

• Authority of the operator
• Authority where the occurrence took place

For example, Air France find a leaking package of radioactive material in the cargo while on the ramp in Chicago. They inform both the French DGAC and the US FAA.

Note: Looking at the wrong answers, both ICAO and EASA don't get their hands dirty with such matters.

EASA AIR OPS
Regulation (EU) No 965/2012
CAT.GEN.MPA.200 Transport of dangerous goods

(e) The operator shall, in accordance with the technical instructions, report without delay to the competent authority and the appropriate authority of the State of occurrence in the event of:

(1) any dangerous goods accidents or incidents;
(2) the discovery of undeclared or misdeclared dangerous goods in cargo or mail; or
(3) the finding of dangerous goods carried by passengers or crew members, or in their baggage, when not in accordance with Part 8 of the technical instructions.