A car bomb killed at least four people in the southern suburbs of Beirut on Thursday, a grim start to the new year as escalating tension between Sunnis and Shiites threatens to propel Lebanon into full-blown sectarian conflict.

The bombing in the Shiite neighborhood of Haret Hreik, which is controlled by Lebanon’s Hezbollah movement, was the second in the capital within a week, affirming an increasingly frequent pattern of retaliatory bombings in recent months.

Ambulances rushed victims away from the scene of the blast — on a residential street lined with grocery stores and cafes — while soldiers searched parked cars amid fears of a second explosives-rigged vehicle.

The increasingly familiar scene of shattered glass and twisted metal left little doubt that Lebanon’s slide toward conflict is accelerating as the country becomes embroiled in the broader sectarian rivalries threatening to engulf the region.

While Iran-backed Hezbollah is deeply entangled in the Syrian conflict, having sent thousands of fighters to shore up forces loyal to Syrian President Bashar al-Assad, Sunni Lebanese fighters have also crossed the border to back the rebels, leaving Lebanon polarized.

Thursday’s explosion did not appear to target any of the many Hezbollah offices or institutions in the area, but it was clearly intended to send a political message.

It came just six days after the assassination of a prominent Sunni politician in a car bombing in downtown Beirut, which many of his political allies blamed on Hezbollah. Tensions were then exacerbated Wednesday with reports of the arrest of Majid bin Muhammad al-Majid, a Saudi national who heads the Abdullah Azzam Brigades. The local al-Qaeda affiliate had asserted responsibility for several attacks on Shiite targets, including a November bombing at the Iranian Embassy that killed 23 people.

“We aren’t afraid, and we aren’t leaving,” said 25-year-old Rasha Moussawi, who said she was thrown off her feet by the blast as she went to buy food, but escaped with minor injuries. “Even if there are 100 explosions, we will stay here for Hasan Nasrallah,” she said, referring to Hezbollah’s leader.

Speaking from the blast site, Health Minister Ali Hassan Khalil confirmed that at least four people had died. Seventy-seven were injured, 10 critically, he said.

Civilians have borne the brunt of the accelerating cycle of violence. Outside Bahman Hospital, where many injured were taken, a family clutched each other in grief after hearing of the death of a 27-year-old woman who relatives said had been out buying vegetables when the bomb exploded.

“Oh, God! Oh, God!” her sister screamed, collapsing to the floor.

Security officials said they were investigating whether the blast was a suicide bombing after human remains were found inside the remnants of the vehicle. The army said the car, a dark green Jeep Grand Cherokee, was packed with 64 pounds of explosives.

It was the third bombing in less than a year to hit Beirut neighborhoods that support Hezbollah. In August, meanwhile, 47 people were killed in a double car bombing targeting two Sunni mosques in the northern city of Tripoli.

An intensifying rivalry between Iran and Saudi Arabia, compounded by the tensions ignited by the war in neighboring Syria, has aggravated long-standing disputes between Lebanon’s Sunni and Shiite political factions, said Rami Khouri, head of the Issam Fares Institute at the American University of Beirut.

“Lebanon has become part of the wider regional confrontation, which is manifesting itself in these local bombings,” he said. “There is open warfare between these two groups, though who is setting them off we don’t know.”

No one immediately asserted responsibility for Thursday’s attack, but a Hezbollah lawmaker linked it to the turmoil in Syria.

“For us, this is an attack intended to destroy Lebanon,” Bilal Farhat told the local al-Jadeed television station, referring to the bombers. “They want Lebanon to be in chaos, with death and destruction, like Syria.”

Clashes immediately broke out in Tripoli after the blast, killing at least one person, according to the official National News Agency. The city has been the scene of frequent battles between pro- and anti-Assad gunmen.

“The disputes taking place on the ground in Lebanon are bigger than the internal security forces can solve,” the country’s caretaker interior minister, Marwan Charbel, warned Thursday.

Lebanon’s political factions have been unable to agree on the composition of a new government since April, leaving the country largely leaderless, and a new push by the biggest Sunni faction to form a cabinet independently of Hezbollah has drawn threats of action from the Shiite movement.

Adding to tensions was a pledge this week from Saudi Arabia to donate $3 billion to the Lebanese army, the only institution capable of challenging Iran-backed Hezbollah, the country’s most powerful military force.

“We hope that it doesn’t get worse,” said Ali Halbawi, 35, who lives a block from the bomb site. “But if the beginning of the year is something to judge by, then it is going to get worse.”

Suzan Haidamous and Ahmed Ramadan contributed to this report.