Greece has spurned its European allies and Washington by blocking European Union sanctions on an Iranian bank the U.S. accuses of financing terrorism, officials familiar with the move say.

Athens’s action last month marked the first time a European country has picked apart the sanctions regime meant to remain in place after the July 2015 nuclear accord with Tehran. The regime is designed to constrain Iran’s ability to resume illicit activities and pressure it to stick by the rules.

Prime Minister Alexis Tsipras’s government undertook to end the sanctions on Bank Saderat, a partly state-owned company that runs Iran’s largest banking network, as Athens seeks to rebuild close economic ties with Iran, a key source of cheap energy for the country in the past. Athens has also been critical of EU sanctions on Russia.

But the stance is potentially risky for Greece, which will host President Barack Obama this month. U.S. Treasury Secretary Jacob Lew warned last year that any firm that deals with Bank Saderat “will risk losing its access to the U.S. financial system.”

For that reason, the end to the EU’s formal sanctions on the Iranian bank is unlikely to immediately improve its prospects in Europe, where financial institutions have so far been reluctant to do business even with Iranian entities that were freed from sanctions under last year’s accord.