The attack was retaliation for militant attack on a military parade last month in the southwest Iranian city of Ahwez. Twenty-five people died in an attack on the parade that commemorated the 1980-1988 war between Iran and Iraq.
Iran initially blamed the attack on Arab separatists backed by the United States and its Gulf allies, but Iranian supreme leader Ayatollah Ali Khameni said Monday jihadists operating in former IS strongholds in Iran and Syria were tied to the attack.
Iranian President Hassan Rouhani promised a "crushing" response.
Six medium-range ballistic ground-to-ground missiles struck areas in eastern Syria in the retaliatory attack Monday, and were followed by an attack by at least seven drones. Iranian news agency Fars reported "the headquarters of Takfiri terrorists" was the target of the missile strikes.
Positions east of the Euphrates River in Syria were hit, near the town of Albu Kamal. U.S. troops are stationed near the town, but officials said they were not the target of the missile or drone strikes Monday.
Iranian news agency PressTV reported the missiles were fired from sites in western Iran, 355 miles away from their target.