Gunmen and suicide bombers attacked Iran's parliament and the shrine of its revolutionary leader on Wednesday, killing at least 12 people, wounding dozens and igniting an hours-long siege at the legislature that ended with four attackers dead.

Pirhossein Kolivand, the head of Iran's emergency department, was quoted by Mizan Online, an Iranian state-run news website, as saying that 12 people were killed and 42 wounded in the two attacks.

The militant Islamic State group claimed the attacks, marking the first time IS has taken responsibility for an assault in Iran. The militants are at war with Iranian-backed forces in Syria and Iraq.

The attacks began mid-morning when assailants armed with Kalashnikov rifles stormed the parliament building. One of the attackers later blew himself up inside, where a session had been in progress, according to a statement carried by Iran's state TV.

Iranian Deputy Interior Minister Mohammad Hossein Zolfaghari told Iran's state TV the apparently male attackers wore women's attire.

The semi-official Tasnim news agency later reported the siege had ended with four of the attackers killed.

The IS group's Aamaq news agency released a 24-second video purportedly shot inside the parliament building during the siege. 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 says in Arabic: "Do you think we will leave? We will remain."

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.

An Associated Press reporter saw several police snipers on the rooftops of buildings around the 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 realised 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 during the siege.

Iranian Parliament Speaker Ali Larijani called the attack a cowardly act. “Iran is an active and effective pillar in the fight against terrorists and they want to damage it,” he said.

Soon after the parliament attack, a suicide bomber and other assailants targeted the shrine of Ayatollah Ruhollah Khomeini located just outside the capital, Tehran, according to Iran's official state broadcaster.

It said a security guard was killed and that one of the attackers was killed by security guards. A woman was also arrested.

Khomeini, who died in 1989, is a towering figure in Iran. He led the 1979 Islamic revolution that toppled the Western-backed shah and became Iran's first supreme leader.

An Associated Press reporter saw security forces, some uniformed and others in plainclothes, around the large and ornate shrine.

IS often claims attacks around the world, even when links to the group cannot be confirmed and appear dubious.

Iranian security officials have not said who they suspect is behind the attacks, though state media has referred to the attackers as “terrorists.”

The unusual attacks prompted the Interior Ministry to call for an urgent security meeting. Officials at one point urged people to avoid using public transportation until further notice.

Khamenei labels attacks 'fireworks'

Iran's supreme leader, Ayatollah Ali Khamenei, dismissed Wednesday's bomb and gun attacks in Tehran as mere “fireworks” that would not weaken the country's fight against terrorism, state TV reported.

“These fireworks have no effect on Iran. They will soon be eliminated ... They are too small to affect the will of the Iranian nation and its officials,” he said.

Khamenei added that Iran, which is helping Syrian President Bashar al-Assad fight rebels that include Islamic State fighters, had prevented worse attacks through its foreign policy.

“If Iran had not confronted terrorists where the core of this sedition is, it would have faced more attacks in Iran,” he said.