Iran broke its silence over the killing of dissident Saudi journalist Jamal Khashoggi, blasting Saudi Arabia and suggesting U.S. support for Riyadh had emboldened operatives in the country to undertake the crime.

Saudi Arabia acknowledged over the weekend that Mr. Khashoggi was killed early this month inside its consulate in Istanbul, claiming the journalist—a prominent critic of the Saudi ruling family—died in a brawl with Saudi officials sent to discuss his return to the kingdom. Turkish President Recep Tayyip Erdogan has...