Twin suicide bombings struck a southern Beirut suburb that's a stronghold of the Shiite Hezbollah group on Thursday evening, killing at least 43 people and wounding scores more in one of the deadliest attacks in years in Lebanon.

The attack was quickly claimed by the extremist group ISIS, which is fighting in neighbouring Syria and Iraq but has until now not had a recognized affiliate in Lebanon, though the tiny Mediterranean country has in the past years faced deadly spillovers from the civil war next door.

The explosions hit minutes apart during rush hour in an area of southern Beirut called Burj al-Barajneh, a stronghold of the militant Hezbollah group. Hezbollah has been fighting in Syria along with Syrian President Bashar al-Assad's forces. The area has been hit in the past and Sunni militant groups have threatened to carry out more such attacks.

Along with the 43 killed, the bombings also wounded 239 people, the Health Ministry announced.

RAW: Chaos after 2 bombs explode in Beirut1:04

It was not immediately clear how many attackers were involved. According to a Lebanese security official, the first suicide attacker detonated his explosive vest outside a Shiite mosque, while the second blew himself up inside a nearby bakery.

An apparent third attacker was found dead, his legs blown off while he still wore an intact explosives belt, said the official, speaking on condition of anonymity in line with regulations. The official speculated that the third may have been killed from the explosion of the second suicide bomber, as he was reportedly close to that blast.

The Al-Mayadeen TV also reported there was a third would-be suicide attacker, and showed a video of a bearded young man who wore an explosives belt. The report said he was killed before he was able to detonate his explosives.

At the scene of the blasts, residents showed reporters what they said were metal pebbles that are usually put inside an explosive belt to inflict maximum casualties.

"They targeted civilians, worshippers, unarmed people, women and elderly, they only targeted innocent people," Hezbollah official Bilal Farhat told The Associated Press, calling it a "satanic, terrorist attack."

Hospitals in southern Beirut were calling on people to donate blood and appealed on residents not to gather at the hospital gates so that ambulance and emergency staff could work unhindered.

'Massacre'

For more than an hour, ambulances struggled to ferry the wounded and the dead from the neighbourhood while Lebanese troops and Hezbollah gunmen cordoned off the area, preventing anyone from getting close to the site of the two blasts, which were less than 50 metres apart.

"There is a massacre inside and we will not let you take photos," a Hezbollah member screamed at an Associated Press photographer at the scene shortly after the explosions. An hour later, ambulance sirens could still be heard in Beirut streets.

Hezbollah also called on people to leave all coffee shops in the area, which are usually packed with people, and urged residents to inform the group about any suspicious moves.

Prime Minister Tammam Salam condemned the "cowardly criminal act," urging the Lebanese to unite. U.N. Special Coordinator for Lebanon Sigrid Kaag also denounced the "heinous attack," stressing the need for those responsible to be brought to justice and saying that the international community was standing by Lebanon.

ISIS posted its claim of responsibility for the bombings on social media pages linked to the Sunni militant group. The claim could not be independently verified but it was similar to other ISIS claims.

ISIS said the attack was carried out by detonating an explosives-laden motorcycle close to a gathering of Shiites — a likely reference to the mosque — and followed by a suicide bomber wearing an explosives vest. The statement made no mention of a third would-be bomber.

"Let the Shiite apostates know that we will not rest until we take revenge in the name of the Prophet [Muhammad]," the IS claim said.

SYRIA-CRISIS/HEZBOLLAH-BLAST

At least 43 people were killed in two suicide bomb blasts in a crowded district in Beirut's southern suburbs, a stronghold of the Shi'ite Muslim militant movement Hezbollah. (Hasan Shaaban/Reuters)

2nd Beirut attack by ISIS 

Thursday's attack shattered a period of relative calm in Lebanon. It was the first such large-scale bombing since mid-2014, and comes amid much political upheaval in the country. It was also the deadliest attack in Lebanon since Aug. 23, 2013, when two car bombs exploded outside two Sunni mosques packed with worshippers in the northern city of Tripoli, killing 47 people and wounding hundreds.

Lebanon has been without president for over a year. The country has seen major protests in the past few months over the government's inability to agree on a solution for a festering garbage crisis, and parliament has not functioned properly for years.

A spate of similar bombings in 2013 and 2014 targeted Hezbollah strongholds in retaliation for the group's involvement in the Syrian civil war, which has angered Sunni groups across the Middle East.

As for the ISIS, this is the second attack claimed by IS so far in Beirut, after a January 2014 bombing in the district of Haret Hreik, also a Shiite neighbourhood in the Lebanese capital, according to the U.S.-based SITE Intelligence Group, which tracks militant messaging on the Internet.

Among those killed in Thursday's blasts were two staffers of the American University of Beirut, according to a memo circulated to the AUB community. The memo did not give the names of the staffers or other details.