Twelve people were killed on Wednesday in a double attack on the Iranian parliament in Tehran and the mausoleum of the founder of the Islamic Republic, Ayatollah Khomeini.

Isis have taken responsibility for the killings.

In the simultaneous attacks, a group of four gunmen stormed the parliament building, which was in session, taking hostages and killing 11, before they were killed in a gunfight with security forces. At the Khomeini mausoleum, two attackers, one of them who detonated a suicide vest, killed a security guard. Iranian state media claim that security forces arrested a third group of attackers before they could reach a third target which was not specified.

This is the first time that the Islamic State have openly carried out an attack inside Iran. The Sunni-dominated Isis has long been an enemy of Shi’a elements backed by Iran, but in the main battlefield of Syria it has concentrated in taking over territory from other, less extreme rebel organisations.

Western intelligence experts have been predicting over the last few months that as Isis loses its main strongholds — Mosul, which has now almost entirely fallen to Iraqi government forces and Shi’a militias, and Raqqa in Syria, where a Western-backed Kurdish force is about to launch an attack on the city — it will intensify its operations further abroad.

The attacks last month at the Manchester Arena and on Saturday night at London Bridge were carried out by individuals who had been in contact with Isis members. The attack on Tehran suggests a much higher level of planning.