The double bombing in a Shiite district of Beirut pulled Lebanon further into a conflict that has torn apart the deeply divided country, and came as Assad's troops, aided by Hezbollah militants, captured a key town near the Lebanese border from rebels.
The bombing was one of the deadliest in a series of attacks targeting Hezbollah and Shiite strongholds in Lebanon in recent months.
An al-Qaida-linked group said it carried out the attack as payback for Hezbollah's backing of Assad forces against the mainly Sunni rebels as the Syrian civil war increasingly becomes a confrontation between regional powers.
The Syrian army's border offensive is part of a larger government push that started last month and has seen forces loyal to Assad firmly seizing the momentum in the war, taking one rebel stronghold after another.
The attacks raised fears in Lebanon that Islamic extremists, now on the defensive in Syria, would increasingly hit back in Lebanon. The country is suffering the effects of competing sectarian loyalties.
"People fight outside (Lebanon), but send their messages through Lebanon. With bombs," said a mechanic whose store windows were shattered by the blasts.
The midmorning explosions hit the neighborhood of Janah, a Hezbollah stronghold and home to several embassies and upscale apartments, leaving bodies and pools of blood on the glass-strewn street amid burning cars.
In the chaotic aftermath, volunteers tried to extinguish bodies still aflame from the blast by covering them with their sweaters and blankets.
U.N. Secretary-General Ban Ki-moon condemned the attacks and called on all Lebanese to recognize that "such appalling and indiscriminate acts of violence" target everyone in the country, U.N. acting deputy spokesman Farhan Haq said.
U.S. Secretary of State John Kerry called the bombings "senseless and despicable," and said "our hearts go out to the Iranian people after this violent and unjustifiable attack claimed the life of at least one of their diplomats.
The dead Iranian was identified as Ibrahim Ansari, a 54-year-old diplomat who took up his post a month ago and was overseeing regional cultural activities, said Iranian Ambassador Ghazanfar Roknabadi, speaking to Hezbollah's Al-Manar TV from inside the embassy compound.
Also among the dead was Radwan Fares, a Lebanese national who headed the facility's security, according to a Lebanese official at the embassy who spoke on condition of anonymity because he was not authorized to give official statements.
The first suicide attacker was on a motorcycle with two kilograms (4.4 pounds) of explosives and blew himself up at the embassy's black main gate, damaging the three-story facility, another Lebanese security official said. He also spoke on condition of anonymity because he was not authorized to talk to the media.
Less than two minutes later, a second suicide attacker driving a car rigged with 50 kilograms (110 pounds) of explosives struck about 10 meters (yards) away, the official added.
Previous large-scale attacks on Hezbollah strongholds include an Aug. 15 car bombing in the southern Beirut suburbs that killed 27 people and wounded more than 300. A less powerful car bomb targeted the same area July 9, wounding more than 50.
Senior Hezbollah official Mahmoud Komati said at the scene that the attacks were a direct result of the "successive defeats suffered by (extremists) in Syria."
He described the blasts as a "message of blood and death" to Iran and Hezbollah for standing by Syria, vowing they would not alter their position.
Shiite Iran is the main Mideast backer of Assad's government, believed to be providing it with key financing and weapons.
A Lebanese al-Qaida-linked group, the Abdullah Azzam Brigades, claimed responsibility for the attacks, saying they would continue until Hezbollah withdraws its forces from Syria.
The authenticity of the claim could not be independently verified. It was posted on a militant website and on the Twitter account of Sirajuddin Zurayqat, a spokesman of the Azzam Brigades.
"It was a double martyrdom operation by two Sunni heroes from Lebanon," he wrote.
The group is active in southern Lebanon and has issued claims in the past for rocket attacks into northern Israel. It has also claimed a 2010 bombing of a Japanese oil tanker in the Persian Gulf and a 2005 rocket attack that narrowly missed a U.S. amphibious assault ship docked at Jordan's Aqaba Red Sea resort.
Iran's Foreign Ministry blamed Israel in a phone conversation with his Lebanese counterpart, while Iranian President Hassan Rouhani blamed "extremists motivated by foreigners."
Hezbollah and Syrian officials indirectly blamed Saudi Arabia, the Sunni Arab kingdom that along with fellow Gulf nation Qatar has been a major backer of Syria's rebels.
"Each of the terrorist attacks that strike in Lebanon, Syria and Iraq reek of petrodollars," a Syrian government statement said, a clear reference to oil-rich Gulf Arab countries.
Syria's U.N. Ambassador Bashar Ja'afari accused Saudi Arabia, Israel and Qatar of backing the al-Qaida terrorists who carried out the attack.
Ayham Kamel, an analyst with the Eurasia group in London, said Sunni rebels and their regional supporters "aim to undermine Hezbollah security in its homeland, deter Hezbollah and Iran from aiding the Syrian military, ... and potentially pressure Iran" ahead of this week's nuclear talks in Geneva.
The Saudis have watched with increasing nervousness as President Barack Obama has approved a cautious opening with their archrival Iran, which could reorder strategic priorities.
At the scene of the blasts, blood was puddled on the ground, and debris and tree limbs were scattered over the streets. Associated Press video showed firefighters extinguishing burning vehicles, as well as bodies covered with sheets. A charred motorcycle stood outside the embassy gate.
Rabie Yehya said he and other volunteers grabbed plastic bags from a nearby abandoned plot of land and began filling them with body parts.
"We emptied them and filled them," he said.
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