Addressing concerns that a landmark nuclear deal reached this year could boost Iran’s military power, the Obama administration reassured critics that it would maintain and enforce its remaining tough sanctions against the country.

Yet the U.S. government has pursued far fewer violations of a long-standing arms embargo against Iran in the past year compared to recent years, according to a review of court records and interviews with two senior officials involved in sanctions enforcement.

The sharp fall in new prosecutions did not reflect fewer attempts by Iran to break the embargo, the officials said. Rather, uncertainty among prosecutors and agents on how the terms of the deal would affect cases made them reluctant to commit already scarce resources with the same vigor as in previous years, the officials said. […]