At the same time, a gun and suicide bomb attack targeted the Ayatollah Khomeini mausoleum on the southern outskirts of the Iranian capital. One of the at.
Iranian state media said police helicopters were circling over the parliament building and that all mobile phone lines from inside were disconnected.
Iran is deeply involved in the fight against ISIS, both in Iraq and Syria, and together with Russian Federation is a major backer of the regime of Syrian President Bashar al-Assad.
0815 GMT: Unofficial sources said the forces of the Hostage Rescue Unit of the Islamic Revolution Guards Corps (IRGC) have managed to arrest two terrorists in the mausoleum. 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 group has claimed the attack on Iran's parliament and another on the shrine of Ayatollah Ruhollah Khomeini on Wednesday, which killed at least two and wounded more than 30 people. Iran provides key ground forces against ISIL and other rebel groups in Syria and Iraq.
The Foreign Office said Islamabad stands in solidarity with the people of Iran after Wednesday's attacks, which killed at least 12 people and were claimed by the Islamic State group.
Sunni extremists, including the ISIS, despise Shiite-majority Iran.
Iran has been singled out as a target by jihadist groups, including IS, but has largely escaped attacks within its urban centres.
Lawmaker Elias Hazrati told state television three assailants, one with a pistol and two with AK-47 assault rifles, raided parliament. An armed man had entered at the western entrance of the mausoleum and opened fire before blowing himself up with a suicide vest, the site's head of public relations Ali Khalili told the IRNA news agency.
Iran's semi-official Tasnim news agency says the siege at parliament is over and that four attackers have been killed.
Iranian media earlier reported a shooting inside the parliament building that wounded a security guard, without providing further details.
CNN has not independently verified the number of victims and it was not immediately clear whether the number of dead included the suicide bombers.
Iranian TV said parliament had resumed, and broadcast footage of what it said was the opening session proceeding normally.