The banking restrictions are arguably the most serious form of economic retaliation yet approved by Western powers in response to Russia’s attack on Ukraine. They are aimed at preventing Putin from using his nation’s sizable financial reserves — totaling more than $600 billion — to stabilize the Russian economy in the face of other sanctions and economic measures imposed by the West.

As of June 30 of last year, 32 percent of Russia’s foreign currency reserves were in euros and 16 percent were in U.S. dollars, according to its central bank. About 7 percent were in British pounds, 13 percent in Chinese renminbi and 22 percent in monetary gold. The remainder was held in other currencies.

“In one fell swoop, the U.S. and Europe have rendered Putin’s war chest unusable. … That the U.S. and Europe have done this in unified fashion sends a crystal-clear message that Russia will face dramatic costs so long as Putin’s war of aggression continues,” said Edward Fishman, former Russia and Europe sanctions lead at the State Department. “This action represents a sea change in U.S. and European strategy. Just 72 hours ago, a step like this was unthinkable.”