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.
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.
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."
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.