The U.S. plans to impose major new sanctions on Iran, signaling it won’t ease pressure despite a diplomatic push by European leaders to persuade Washington and Tehran to show restraint.

The administration has ratcheted up economic pressure on Tehran since President Trump pulled the U.S. out of the 2015 nuclear deal in May 2018, hoping to drive Iran to accept a tougher agreement that would end uranium enrichment and curb its regional ambitions.

In...