Emergency personnel remove a body from the site of a twin suicide blasts in Burj al Barajinah in the southern suburbs of Beirut on Nov. 12, 2015. (Anwar Amro/AFP/Getty Images)

Hezbollah militiamen were on high alert Friday after twin suicide bombings left dozens dead and raised fears of wider Islamic State attacks in Lebanon as a deadly spillover from neighboring Syria.

Security personnel clad in fatigues fanned out across Hezbollah strongholds in mostly Shiite southern Beirut, inspecting vehicles and searching residents. At least 43 people were killed and hundreds wounded in Thursday’s blasts, which were claimed by the Islamic State.

The attacks also underscore the mounting strains on Iranian-backed Hezbollah as it grapples with securing its bases in Lebanon while engaging in combat in Syria to support the embattled government of President Bashar al-Assad, a key ally of Russia and Iran.

The Sunni-led Islamic State, which holds significant territory in Syria, opposes Iran’s Shiite leadership and preaches against Shiite Muslims.

The bombings in the densely populated Burj al Barajinah district were the deadliest to hit Beirut in years. Lebanese Prime Minister Tammam Salam issued orders for increased security measures.

“The barbaric crime in Burj al Barajinah didn’t target a certain area or sect, but smudged Lebanon with blood,” Salam said in a statement released by his office, according to Lebanon’s Daily Star newspaper.

In Syria, thousands of Hezbollah fighters are taking part in a new push — coordinated with Syrian government forces, Iranian units and Russian air support — to take back strategic areas of the country captured by mostly Sunni rebels.

Hezbollah’s intervention in Syria — without Lebanese government approval — also has rattled Lebanon’s brittle sectarian relations. Enraged Sunni militants in Lebanon and Syria carried out a string of bombings against Lebanese Shiites last year.

“Hezbollah said it went to Syria to protect Lebanon, but what it’s doing is bringing the Syrian war to Lebanon instead,” said Nawfal Daou, a Lebanese political commentator who is critical of Hezbollah.

Thursday’s attacks appeared to be an attempt to further complicate Hezbollah’s balancing of domestic and foreign interests, analysts said.

Already, Hezbollah has sustained heavy losses in Syria. Some estimates put Hezbollah’s dead at more than 1,000 in a civil war that has killed more than 250,000 people and forced millions to flee to surrounding countries, including Lebanon.

Hezbollah also diverts massive quantities of resources to possible future clashes with its traditional enemy, Israel.

“With these suicide bombings, the militants are trying to force Hezbollah to make a decision over priorities: secure Lebanon or fight in Syria,” said Labib Kamhawi, an analyst based in Jordan.

Over the past year, Hezbollah fighters carried out attacks along the Lebanon-Syria border to stop Sunni militants from directing a spate of bombings in Shiite areas of Beirut.

Those operations by Hezbollah received the tacit approval of Lebanon’s Sunni politicians, who chose cooperation with Hezbollah because of the gathering threat from the Islamic State.

Thursday’s attacks raise questions about the durability of that coordination in Lebanon. They also appeared to stoke the sectarian passions that Hezbollah has sought to tamp down within its Shiite constituency.

“We can’t just sit here and wait to be attacked again. Hezbollah must do something and let us all go to Syria to fight and kill these people,” said Hussein, 35, a Shiite resident of a southern suburb who works for a company that paints houses. He declined to give his full name, citing concerns over publicly criticizing Hezbollah.

Some residents of southern Beirut called for expelling Syrian refugees from Lebanon, alleging a fifth-column threat.

“We want Hezbollah to kick out all the Syrians from Lebanon,” said Ibrahim, 28, a father of a young child and a resident of Burj al Barajinah. “The Shiite-Sunni war has exploded, and there is no need to cover up its existence or try to claim that it doesn’t exist.”

A number of residents in south Beirut said they received threatening messages on the WhatsApp communications platform shortly after the bombings, with the voice of an apparent Sunni militant in Syria vowing more attacks in Lebanon.

On social media sites, the Sunni Abdullah Azzam Brigades group, which has carried out multiple attacks on Shiite interests in Lebanon, praised the bombings.

“The southern suburbs of Beirut, the stronghold of Iran’s party, is drowning in blood after they sent their killers and criminals to Syria,” a spokesman for the group said on Twitter.

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