Taking any of it without due process is a violation of EU laws
It's not only a violation of EU and US laws, but of international law.
I'm not sure what this may be called in the EU, but in the United States there is a law called the Foreign Sovereign Immunities Act, or FSIA. As mentioned above, the US has this law on the books not because it is so incredibly fair, but rather it implemented these protections because they are required by international law.
The FSIA grants immunity to foreign states and their agencies and instrumentalities from the jurisdiction of U.S. federal and state courts, subject to certain exceptions. Those exceptions concern acts of terrorism and full-on hostilities between states. The only time America seized sovereign assets (not froze), was during the George W. Bush years when they confiscated Iraqi government reserves during the war.
Since neither the US, nor any EU state, has declared war on Russia - there can be no legal justification for seizure.
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The bottom line: states enjoy protections above and beyond persons or companies. Large parts of the body of international law, are concerned with enumerating the various protections that states grant each other. This principle is hundreds of years old, but it looks like the West may be prepared to trample over it anyway.