ISIS claimed responsibility for a pair of Wednesday attacks in Tehran in which suicide bombers and teams of gunmen stormed Iran's parliament and the nearby shrine of Ayatollah Ruhollah Khomeini, killing at least 12 and injuring dozens of others.

This is the first attack orchestrated by ISIS in the Islamic Republic, SITE Intel Group reported. It wasn't initially clear if the death count, reported by state broadcaster IRIB, included the attackers.

In a rare and stunning move, ISIS released video from inside the parliament building while the attack was under way. The video, circulated online, shows a gunman and a bloody, lifeless body of a man lying on the ground next to a desk. A voice on the video praises God and says in Arabic: "Do you think we will leave? We will remain, God willing." Another voice repeats the same words. The two appeared to be parroting a slogan used by IS spokesman Abu Mohammad al-Adnani, who was killed in Syria last year.

The parliament assault ended Wednesday morning with all four attackers there being killed.

"Iranian nation moves on, today the fumbling with firecrackers in #Tehran, won't affect the will of our nation," Ayatollah Kahmenei wrote in a tweet.

State Department spokeswoman Heather Nauert said "the depravity of terrorism has no place in a peaceful, civilized world."

One of the terrorists blew himself up inside the parliament building, where a session had been in progress, according to a statement carried by Iran state TV. It quoted lawmaker Elias Hazrati as saying the attackers were armed with Kalashnikov assault rifles.

An Associated Press reporter saw several police snipers on the rooftops of buildings around parliament. Shops in the area were shuttered, and gunfire could be heard. Witnesses said the attackers were shooting from the fourth floor of the parliament building down at people in the streets below.

"I was passing by one of the streets. I thought that children were playing with fireworks, but I realized people are hiding and lying down on the streets," Ebrahim Ghanimi, who was around the parliament building when the assailants stormed in, told The Associated Press. "With the help of a taxi driver, I reached a nearby alley."

Police helicopters circled over the parliament building and all mobile phone lines from inside were disconnected. The semi-official ISNA news agency said all entrance and exit gates at parliament were closed and that lawmakers and reporters were ordered to remain in place inside the chamber.

State TV reported four attackers were involved in the parliament attack.

Iran's official state broadcaster said a security guard was killed and four people wounded in the shrine attack. It said one of the attackers at the shrine was killed by security guards and that a woman was arrested. It described the shrine attackers as "terrorists" and said one carried out a suicide bombing, without providing further details.

In addition to being lethal, the attack on the shrine of Khomeini is symbolically stunning. As Iran's first Supreme Leader, Khomeini is a towering figure in the country and was its revolutionary leader in the 1979 ouster of the shah.

The Associated Press contributed to this report