LONDON (Reuters) - Suicide bombers and gunmen attacked the Iranian parliament and the mausoleum of Ayatollah Khomeini in Tehran on Wednesday, killing at least 12 people in a twin assault which Iran's Revolutionary Guards blamed on regional rival Saudi Arabia.
Islamic State said it was responsible, the first time the hardline Sunni Muslim militant group has claimed attacks in the tightly controlled Shi'ite Muslim country.
But other Sunni militant groups in Iran have long carried out periodic attacks on military and civilian targets, aiming to highlight what they say is discrimination against Iran's Arab minority and Sunni ethnic groups.
Iran has stepped up its crackdown against members of such networks in recent years, with mass arrests and death sentences.
The Arab minorities who mainly live in Iran's oil-rich southwestern Khuzestan province, and Sunni Baluchis in poor Sistan-Baluchestan in the country's southeast, say they are deprived of decent living standards and some civil rights.
The main Sunni Iranian separatist groups increasingly see themselves as part of a larger struggle between Shi'ite Iran and the Sunni-ruled Arab states across the Gulf, which back opposing sides in the Syrian civil war, Iraq and Yemen.
Tehran claims Saudi Arabia is funding most of these groups, a charge Riyadh denies. Below is the list of main Sunni militant groups in Iran:
Jaish al-Adl (or the Army of Justice) is a Sunni militant group that has carried out several attacks on Iranian security forces mainly in Sistan-Baluchestan.
Jaish al-Adl claimed responsibility in April for attacks that killed 10 Iranian border guards near Pakistan.
Iran says the group has safe havens in Pakistan and has warned it will hit their bases there if Islamabad does not confront them.
Jundullah (or Soldiers of God), a Sunni separatist group, has claimed responsibility for bombings in the last decade that have killed scores of Iranians, including senior commanders of the Revolutionary Guards.
The group, which also has bases in Pakistan, is believed to be significantly weakened after its leader Abdolmalek Rigi was captured and executed by Iran in 2010.
Jundullah also targets Pakistan's minority Shi'ite community and pledged allegiance to Islamic State in 2014.
Ansar al-Furqan (or Partisans of the Criterion) is a Baluchestan-based Sunni jihadi group that has threatened to carry out suicide attacks on Iranian economic and military centres in revenge for the government executing Sunni prisoners.
The group was formed when Sunni Baloch insurgent group Harakat Ansar Iran (Movement of the Partisans of Iran), active in Sistan-Baluchestan since 2012, merged with another local Sunni group, Hizbul-Furqan, in 2013.
The move aimed to strengthen their fight against Tehran's authorities.
The Mohiuddin al Nasser Martyrs Brigade, the armed wing of the Arab Struggle Movement for the Liberation of Ahwaz (ASMLA) has claimed responsibility for several attacks in Iran.
The group killed two members of the Revolutionary Guards in January in Khuzestan province.
The group has also claimed it has blown up several oil pipelines in Khuzestan in recent years.
Ahvazi Arabs are a minority in mainly ethnic Persian Iran, and some see themselves as under Persian occupation and want independence or autonomy.
Reporting by Bozorgmehr Sharafedin; Editing by William Maclean and Catherine Evans