The attacks in the Iranian capital striking the country's parliament and a shrine killed 17 people.
Iran's Intelligence Ministry said Wednesday's massacre was perpetrated by Iranians loyal to the Islamic State militant group who fought in Iraq and Syria.
The Islamic State militant group claimed responsibility for twin attacks in the Iranian capital that killed 13 people.
The statement follows the first major ISIS attack on Iranian soil.
After being attacked, a coalition of forces backing the Syrian government and fighting the Islamic State militant group has warned the U.S. not to cross "red lines."
The group said two of its fighters carried out the attacks, which killed seven people.
The attacks in the Iranian capital left at least seven dead, according to local reports.
The State Department's Stuart Jones appeared took a 20-second-long silent pause when a reporter asked him about U.S.' backing of democracy and Saudi Arabia
Muslim-majority countries have different ways of determining the start of Islam's holiest month.
Speaking in Jerusalem the U.S. president also said Iran needed to cease funding of terror organizations.
Tehran and Riyadh are regional arch-rivals which accuse each other of sponsoring fundamentalist militias aligned to their competing sects of Islam.
Rouhani had a resounding win but angered hardliners with his comments during the campaign, and they are likely to create problems for him.
Syrian and Iranian concerns regarding the U.S. and Jordan's nearby military drills were heightened when U.S. troops were seen with rebels in Syria.
U.S. officials say Iran recently tested its supercaptivating Hoot torpedo, capable of traveling 250 miles per hour by evaporating the water around it.
The industrial disaster in north-eastern Iran is the most deadly in the Islamic Republic for decades.
Iran's defense minister was responding to Saudi comments about the "battle" for influence in the Middle East.
The Syrian government says it destroyed its stockpiles in 2014.