Air Serbia cancelled a flight from Belgrade to Vienna, but now refuses to pay either meal/accommodation expenses or compensation, despite already having promised in writing and over the phone to pay expenses. We appealed to the Agency for Passenger Rights, Austria's national enforcement agency for passenger compensation claims, but they replied that the matter is out of their jurisdiction. (Flights into the EU on non-EU carriers are generally not covered by EU Regulation 261/2004.)
However, Air Serbia maintains its own statement of passenger rights* and also mentions that these rights are prescribed by Serbian law, so we may still be able to pursue the matter in Serbia. What government agency is responsible for enforcing passenger compensation rights in Serbia, and how can we contact them to lodge a complaint? If there is no such government agency, is there a private claims processor that we can use? (There are a lot of such processors in the EU, such as ClaimCompass, but all the ones we checked deal only with claims covered by the EU regulations, not the Serbian ones.)
* Note that due to Air Serbia's broken website, you may need to follow that link twice before the correct page appears. It seems that first-time visitors to the website are always redirected to its home page.
UPDATE: In case anyone is interested, on 19 April 2020 we submitted our claim by e-mail to the Civil Aviation Directorate of the Republic of Serbia, as suggested in the answer by Crazydre. By 6 May 2020 we had not received a response, so on that day we sent a copy of the complaint via registered post. It's been over three months since the initial submission, and we still haven't had any response from the Directorate.