BEIRUT - Iran fired missiles on Sunday into eastern Syria, aiming at the bases of militant groups it holds responsible for attacks in Tehran which left 18 dead last week, Iran's Tasnim news agency reported.Iranian Revolutionary Guards launched the mid-range ground-to-ground missiles from western Iran into the Deir al Zour region of eastern Syria, killing a "large number" of terrorists and destroying their equipment and weapons, it said.The missiles targeted the "headquarters and gathering centers of Takfiri terrorists supporting and building car bombs," it said.Reuters could not independently verify the report.Military leaders and officials in Iran, a predominantly Shi'ite country, often refer to Sunni Muslim radicals as Takfiris.The Revolutionary Guards are fighting in Syria against militant groups who oppose President Bashar Assad.The attack last week, which included shootings and at least one suicide bombing, was on Iran's parliament and the shrine of Ayatollah Ruhollah Khomeini, the founder of the Islamic Republic."The spilling of any pure blood will not go unanswered," the Revolutionary Guards said in the statement quoted by Tasnim.Islamic State issued a statement claiming responsibility for the Tehran attack.Senior Iranian officials, however, have pointed a finger at Saudi Arabia, Iran's Sunni regional rival.In another event taking place on Sunday in the region, the Syrian army said that the US-led coalition downed a Syrian army jet in the southern Raqqa countryside during a Syrian combat mission against Islamic State militants.An army statement released on Syrian state television said the plane crashed and the pilot was missing. It said the incident took place on Sunday afternoon.This is a developing story.