At least 22 people are reported to have been killed in two explosions which hit the Iranian embassy in the Lebanese capital Beirut in quick succession.
The Iranian cultural attache Ebrahim Ansari was among the dead. Officials said the death toll could rise.
TV images showed burning cars, bodies on the street and damaged buildings.
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 Iranian ambassador to Beirut confirmed Mr Ansari’s death to Hezbollah’s al-Manar TV, but said 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, he said.
The ambassador blamed Israel for the attack – an accusation Israel swiftly rejected.
The Syrian conflict has increased sectarian tensions in Lebanon.
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.
A BBC correspondent in Beirut says those responsible wanted to send a clear message to Iran and Hezbollah.
Tehran and the Shia militant group are key backers of the Syrian government, which is currently trying to cut off one of the Syrian rebels’ last remaining supply routes across the Lebanese border.
The main gate to the embassy was blown out and there was damage to the three-storey building, the Associated Press reported.
AP and al-Manar quoted sources saying that the first blast was caused by a suicide bomber, while the second was a car bomb, which did much more damage.
However, this has not been officially confirmed.
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.
Hezbollah fighters were instrumental in a strategic victory by Syrian government forces in Qusair, close to the border with Lebanon, in early June.