Beirut bomb iran embassy
A man walks near burning cars at the site of explosions near the Iranian embassy in Beirut (Reuters)

A jihadist group linked to al-Qaida has claimed responsibility for a twin suicide bombing in Beirut that killed more than 20 people including an Iranian diplomat.

The Abdullah Azzam Brigades, a Sunni militant group, said it carried out the attack against Tehran's diplomatic compound in the Lebanese capital, in what the local government described as a "cold blooded act of terrorism".

The brigades' leader, Sheikh Siraj al-Din Zureiqat said the bombing came in retaliation to Iran's support to the regime of Bashar al-Assad in the neighbouring Syria.

The Iranian cultural attaché, Sheikh Ibrahim Ansari died from injuries reported in the double blast that killed another 22 people and injured 146, officials said.

"These innocents were killed in an act of cold blooded terrorism," former Lebanese Defense Minister Albert Mansour told al-Arabiya.

"This attack will add fuel to the fire of sectarianism in the country and hostility between Shiite and Sunni Muslims."

The Syrian civil war has polarised confrontation between Sunni and Shia across the Middle East and violence has spilled over into Lebanon, a country that was itself torn apart by a bloody civil war from 1975 to 1990.

Iran is a major supporter of Lebanese Shia militant group Hezbollah and the embassy is located in the southern neighbourhood of Janah, which is considered a Hezbollah stronghold.

Hezbollah and Iran are actively supporting the Syrian regime in the civil war, led prevalently by Sunni rebel groups, that has seen the growing influence of Islamist extremist fighters.

Hezbollah militiamen are fighting alongside government troops in Syria while Tehran has admitted sending military personnel to advise and train Damascus forces.

Hezbollah officials threatened further retaliation. "We tell those who carried out the attack, you will not be able to break us," Hezbollah lawmaker Ali Mikdad told Al-Mayadeen TV. "We got the message and we know who sent it and we know how to retaliate."

The two bombs went off a few minutes one from the other, blowing up the embassy's main gate and damaging several nearby buildings.

According to an Iranian armed guard patrolling the premises, the first explosion was detonated by a suicide bomber who drove a motorcycle to the embassy's entrance and blew himself up.

The second blast was said to be a car bomb, also detonated by a suicide attacker.

Paramedics rushed the scene to help the wounded and heavy security forces were deployed.

Tehran had initially pointed the finger against Israel. Foreign ministry spokeswoman Marzieh Afkham told official Irna news agency the bombings were "an inhuman crime and spiteful act done by Zionists and their mercenaries."

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A Lebanese Army soldier patrols on a military vehicle in front of the Iranian embassy in Beirut after a twin suicide bombing (Reuters)