1. Council of the European Union. (2018). EU Best Practices for the effective implementation of restrictive measures. Document 8519/18.
Page 20, Section V, Paragraph 69: "Operators should, without delay, inform the competent authority of the Member State where they are resident or located of any assets frozen in implementation of the restrictive measures in force and of any authorisation granted, in accordance with the relevant provisions of the legal acts." This directly supports the requirement to inform authorities.
2. Council Regulation (EU) No 269/2014 of 17 March 2014. Concerning restrictive measures in respect of actions undermining or threatening the territorial integrity, sovereignty and independence of Ukraine.
Article 9, Paragraph 2: "Natural or legal persons, entities or bodies shall immediately supply any information which would facilitate compliance with this Regulation, such as information on accounts and amounts frozen in accordance with Article 2, to the competent authorities of the Member States where they are resident or located..." This article establishes the legal obligation to supply information about frozen assets to competent authorities.
3. ACAMS. (2023). Certified Global Sanctions Specialist (CGSS) Study Guide.
Chapter 4: Sanctions Evasion and Investigations: The guide details the process following a confirmed sanctions match, which includes blocking/freezing the transaction or account and then promptly filing a report with the relevant regulatory body (e.g., OFAC in the US, or the respective NCA in the EU). This confirms that reporting is a mandatory step after freezing.