Two explosions near the Iranian embassy in Beirut have killed at least 23 people, wounded more than 150, including an Iranian diplomat, and caused extensive damage to one of Lebanon's most heavily guarded buildings.

The attack shattered more than two months of relative calm in Lebanon and was cast by some officials as another spillover from the devastating war in neighbouring Syria, in which Iran, along with other regional powers, has taken a prominent stake.

One of the explosions is thought to have targeted a convoy arriving at the embassy, which contained cultural attaché Sheikh Ibrahim al-Ansari. Ghazanfar Rokanabadi, Iran's ambassador to Lebanon, confirmed Ansari's death, Iranian semi-official news agency Fars said. Lebanese officials also said he had been killed.

Among those killed were embassy guards, who eyewitnesses said had tried to stop a suicide bomber riding a motorbike near the building's gates, which were destroyed in the attack. The first bombing is thought to have been a prelude to a more substantial explosion about a minute later. A large crater near the embassy gate revealed the destructive force of the bomb, which is thought to have been hidden in a car.

Gunfire was heard in the minutes after the blasts as security forces tried to hold back bystanders and allow a cavalcade of rescue vehicles to enter the Bir Hassan area on the western edge of Hezbollah's Beirut stronghold.

The Lebanese militia has been on high alert in its heartland since August when the second of two explosions within weeks ravaged a nearby civilian area, killing scores.

The Shia Islamic leadership of Iran and its Hezbollah ally are strongly supportive of Bashar al-Assad's regime in Syria, while those fighting against it are almost all Sunni Muslims – many of them homegrown Syrians, but also including jihadists who have travelled to Syria to fight the regime and its backers.

As state power has crumbled in Syria, sectarian faultlines have been stretched in Lebanon, where, despite their 1,500-year-old schism, the two main sects of Islam have more or less co-existed since both countries were formed from the ruins of the Ottoman empire.

But such an accommodation is increasingly being tested here, and elsewhere in the region, where the two sects live in proximity. Iraq has witnessed almost daily bombings for the past six months, nearly all of them carried out by extremist Sunni groups who openly state they are trying to reignite the sectarian war that raged there from 2006-07.

Both Iran and Hezbollah have played lead roles in recent advances by Syrian forces around Aleppo in the north and in rebel-held land south of Damascus. Hezbollah is also believed to be at the vanguard of a regime offensive in the Qalamoun mountains just to the east of the Syrian border, which looms as a significant battleground in the overall fight for control of the country.

With the war raging and regional tensions continuing to reverberate, Syrian opposition political leaders have yet to commit to a summit that aims to bring the crisis to a negotiated end. Opposition leaders say they remain opposed to Iran taking part and to Assad playing any future role in Syria.

Iran's ambassador blamed Israel for the attack. The Israeli prime minister, Benjamin Netanyahu, remains vehemently opposed to negotiations between Iran, the US and Europe over the fate of Tehran's nuclear programme, which Iran insists is for civilian purposes but Tel Aviv counters is a cover to make nuclear weapons that will be used to threaten it.