The Islamic State in Iraq and Syria (ISIS) claimed responsibility for the double attack at the Iranian parliament and the shrine of Ayatollah Khomeini in Tehran on Wednesday.

"Fighters from the Islamic State (ISIL) attacked the Khomeini mausoleum and the parliament building in Tehran," the terror group said via its Aamaq news agency.

The group's news agency said in another statement that two suicide bombers carried out the attacks on the shrine of Khomeini, according to Reuters.

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The Aamaq news agency also released a video reportedly shot inside the parliament, The Associated Press reported.

In the video, a voice is heard saying in Arabic: “Do you think we will leave? We will remain, God willing.”

The attacks, which started midmorning on Wednesday, killed at least 12 people and wounded dozens more, the AP reported.

Attackers armed with rifles went into the parliament building, and one detonated a suicide vest inside amid the ongoing siege, according to state-run media.

Iranian Parliament Speaker Ali Larijani said Iran is an "active and effective pillar in the fight against terrorists and they want to damage it."