President Obama issued a formal determination Friday that global oil supplies are sufficient to move ahead with toughened sanctions against Iran.

Obama, acting under a congressionally mandated deadline, issued a finding that’s a precursor to sanctions against foreign banks that purchase Iranian oil. The toughened sanctions are designed to reduce Iran’s oil revenues and isolate its central bank.

The determination that there are enough supplies to allow nations that import Iranian oil to find other sellers will enable sanctions to take effect as soon as June 28.