U.S. sanctions intended to squeeze an Iranian paramilitary force may have unintended consequences for humanitarian efforts in the country because of its targeting of a bank, lawyers and analysts say.

They also could signal a harder-line approach toward Iran as the U.S. prepares to restore the final tranche of its sanctions lifted following the 2015 nuclear accord that target key economic engines, such as oil and petrochemicals. President Donald Trump said in May when exiting the nuclear deal that the sanctions would phase...