At least 22 people have been killed and more than 140 injured in a double suicide bombing outside the Iranian embassy in the Lebanese capital Beirut.
The Iranian cultural attache, Sheikh Ibrahim Ansari, is among the dead, according to the Fars news agency.
Iran is a major backer of the Lebanese Shia militant group Hezbollah, which has sent fighters to Syria to back the government of Bashar al-Assad.
The Sunni jihadist group Abdullah Azzam Brigades said it was behind the attack.
The head of the al-Qaeda-linked group described the attack as a "double martyrdom operation carried out by two heroes from the heroic Sunnis of Lebanon".
The bombings were condemned by the United Nations Security Council, and the US described them as "senseless and despicable".
The conflict in Syria has increased sectarian tensions in its smaller neighbour.
Lebanese Sunni Muslim fighters have joined forces with the mainly Sunni rebels in Syria. Some of the rebel groups are affiliated with al-Qaeda.
Syria's President Assad comes from the Alawite sect, a heterodox offshoot of Shia Islam.
The BBC's Paul Wood, in Beirut, says the number of people killed in Tuesday's attack makes it one of the worst in Shia southern Beirut since the war across the border in Syria began.
But more significantly, he says, it is the first attack on an Iranian target.
Analysis
Paul WoodBBC News, Beirut
The number of people killed makes this one of the worst attacks in Shia southern Beirut since the war across the border in Syria began - but it is, most significantly, the first attack on an Iranian target.
"Was this payback for Iran's support for President Assad?" I asked a Hezbollah member of parliament, who'd come to see the damage. "Yes, certainly," he told me, and an action that could light fresh fires in Lebanon, he went on, anxiously.
The attack comes as the Syrian army is carrying out a major offensive to cut off the rebels' last supply routes into Lebanon. For the Syrian rebel movement, these are desperate times - the regime's boot is on their throat, held there with Iranian assistance.
The attack on the embassy has been claimed by a Lebanese group, but one fighting in Syria with the rebels. If it was responsible, the Iranians have many resources inside Lebanon to hit back.
Black ribbon
The Iranian ambassador in Beirut confirmed Mr Ansari's death to Hezbollah's al-Manar TV, saying it was not clear if he had been in the embassy itself or one of the residential buildings nearby.
People gathered at the scene of the two blasts near the Iranian embassy in the neighbourhood of Janah, a Hezbollah stronghold
More than 140 people were wounded by the double blast on Tuesday morning
Mr Ansari had only taken up his post a month ago.
Lebanese officials said the first suicide attacker was on a motorcycle, while the second was in a four-wheel drive vehicle.
The Syrian government condemned the attack, as did UK Foreign Secretary William Hague, who said: "The UK is strongly committed to supporting stability in Lebanon and seeing those responsible for this attack brought to justice."
Continue reading the main story
Syria crisis spills into Lebanon
- May 2012: Fighting between pro- and anti-Assad groups in Lebanese Tripoli and Beirut leave many dead
- August 2012: Deadly sectarian clashes break out in Tripoli
- October 2012: Several people killed in gunfights after the assassination of top security official General Wissam al-Hassan, a Sunni opponent of Damascus
- May 2013: At least 15 people die in another round of sectarian violence in Tripoli
- June 2013: At least 17 soldiers killed in clashes with supporters of radical Sunni cleric Sheikh Ahmad al-Assir in the southern city of Sidon
- 9 July 2013: A car bomb wounds dozens in a Hezbollah stronghold in Beirut
- 15 August 2013: A car bomb kills 27 people and injures hundreds more in a Shia area of south Beirut
- 23 August 2013: More than 40 people killed and 400 injured in two blasts outside mosques in Tripoli
US Secretary of State John Kerry said: "The United States knows too well the cost of terrorism directed at our own diplomats around the world, and our hearts go out to the Iranian people after this violent and unjustifiable attack."
Iran and Lebanon played a football match in Beirut on Tuesday, but without spectators. The Iranian players wore a black ribbon on their jerseys in solidarity with the victims of the attacks.
South Beirut, including the area around the Iranian embassy, is considered a Hezbollah stronghold. It has been hit by several attacks in recent months.
Caretaker Prime Minister Najib Mikati called the attack "a cowardly terrorist act", Lebanese state news agency NNA reported.
"The aim of the blast is to stir up the situation in Lebanon and use the Lebanese arena to convey messages," he said.
This is not the first time the Syrian conflict has spilled over into violence in Lebanon.
On 15 August, 27 people were killed in a car bomb in south Beirut believed to have been targeting a Sunni Muslim cleric opposed to Hezbollah. The cleric was unhurt.
Later in August, more than 40 people were killed in two blasts outside mosques in Tripoli.
Meanwhile, inside Syria, the government appears to be winning its attempt to cut off one of the rebels' last remaining supply routes across the Lebanese border.
The Syrian army has taken control of the town of Qara, meaning it now controls the road linking the coast to the capital.
[http://www.bbc.co.uk/news/world-middle-east-24997876]
Two vehicles from the nearby presidential palace took away the casualties
