The United States exempted seven countries from sanctions targeting the Central Bank of Iran, after Secretary of State Hilary Clinton determined that the nations have significantly reduced their purchases of Iranian oil.

The Obama administration will begin implementing unprecedented sanctions against Iran’s central bank on June 28. The sanctions, signed into law last December as part of this year’s National Defense Authorization Act, authorize the secretary of state to exempt from sanctions foreign banks whose governments have “significantly reduced” their countries’ purchases of Iranian oil.

“Today’s announcement underscores the success of our sanctions implementation,” Clinton said in a press statement issued Monday, June 11. “By reducing Iran’s oil sales, we are sending a decisive message to Iran’s leaders: until they take concrete actions to satisfy the concerns of the international community, they will continue to face increasing isolation and pressure.”

The secretary’s determination follows her decision last March to exempt eleven countries from the sanctions. While the United States has awarded exemptions to many of the world’s largest buyers of Iranian crude – including India, South Korea and Turkey — it  has yet to lift penalties against China, Iran’s most reliable oil customer. The exemptions will need to be renewed every 180 days and are dependent upon continued reductions in purchases of Iranian oil.