ERBIL, Iraq — At least 80 people, many of them Shiite pilgrims on their way home to Iran, were killed on Thursday when an Islamic State suicide bomber detonated a truck filled with explosives at a roadside service station in southern Iraq, local officials said.
The devastating attack came two days after Prime Minister Haider al-Abadi applauded the security forces for protecting the millions of Shiites who have flowed through southern Iraq in recent days for what many consider the world’s largest religious pilgrimage, larger even than the hajj in Saudi Arabia.
In years past, the annual rite known as Arbaeen, a commemoration of the martyrdom of the revered seventh century Shiite figure, Imam Hussein, was a frequent target of Sunni extremist groups like the Islamic State and its predecessor Al Qaeda in Iraq.
Up until the bombing on Thursday, which the Islamic State claimed in a statement, the event had been carried out safely. That was seen as a success for Mr. Abadi and the military, and it was hailed as a sign that the government could keep pushing a major offensive against the Islamic State in Mosul, while protecting religious pilgrims in the south.
The bombing, in Hilla, south of Baghdad, shattered that illusion. The Islamic State clearly remains a potent force, both on the battlefields of Mosul, where fighters are putting up a last stand against Iraq’s elite counterterrorism forces in eastern neighborhoods, and in its ability to carry out more traditional guerrilla attacks.
For nearly six weeks, tens of thousands of Iraqi security force members have been fighting in northern Iraq to retake Mosul, the country’s second-largest city and the last stronghold of the Islamic State in Iraq. Government forces have found themselves bogged down fighting house to house in dense urban neighborhoods, and many civilians have been killed.
A victory in Mosul could mark the end of the Islamic State’s control of territory in Iraq. But the bombing Thursday was a stark reminder that the group will most likely revert to its roots as a guerrilla insurgency and continue to carry out terrorist attacks across the country.
The bombing was the deadliest in Iraq since an explosives-filled truck blew up near a shopping mall in Baghdad in July, killing at least 300 people in the worst terrorist attack in the capital since the American invasion of Iraq in 2003.
That attack alarmed American officials, who worry that efforts to help the Iraqis defeat the Islamic State in Mosul will be squandered if civilians around the country are terrorized by suicide bombings. In recent months, the United States has rushed intelligence advisers and new drones to Iraq to counter car bomb attacks in Baghdad and the south. Before Thursday’s bombing, there had been a noticeable decline in the numbers of attacks.
Falah al-Khafaji, the head of the provincial security committee in Babil Province, where the bombing occurred, said that at least 80 people had been killed and that dozens more had been wounded. Mr. Khafaji said early reports suggested that the bomber had followed a bus of Iranian pilgrims and then detonated his explosives in a crowded parking lot at a gas station-restaurant complex, setting fire to a number of vehicles and fuel tanks.
Another local official, Dr. Nowras Abdulrazak, the head of Babil’s health directorate, said on Thursday that his casualty toll was 75 people dead and 25 wounded. That number was likely to rise, he said.
Most of the victims were Iranians, officials said, but it was unclear in the immediate aftermath of the attack exactly how many. Iranian state television, however, reported that a majority of the victims were Afghan citizens living in Iran.
Iran is the region’s pre-eminent Shiite power, and millions of its citizens annually flock to Iraq, which is also majority Shiite, to visit important religious sites in the cities of Karbala and Najaf.
The Islamic State said in a statement released after the attack that one of its fighters, an Iraqi man, had carried out the suicide bombing. The group vowed to spread the “flames” of the battles underway in Mosul to the Shiite-dominated south, according to a translation released by the SITE Intelligence Group, which tracks jihadist statements.
The Islamic State considers Shiites to be apostates, and it has long carried out a campaign of sectarian killings in Iraq. In its statement, it called Shiites “miscreants” and said, “What awaits them in the immediate future, Allah permitting, is more sorrowful and bitter.”
One of the people hurt in the bombing, Toma Muhan, 31, said on Thursday that just before the explosion, he parked his car at the service station. He was walking away when he was knocked on the ground.
“I felt a strong blast at my back and then severe pain in my leg,” he said from a hospital bed, where he was being treated for shrapnel wounds and burns to his neck and legs. “I remember hearing loud screaming, and then I passed out and woke up later at the hospital.”
A local police officer, Col. Thamir Jhazala, said that the restaurant and gas station had just opened recently, and that security was not good. He cited what he said was a lack of surveillance cameras.
Numan al-Jibouri, who works at the station, said that Thursday was a day off for him, but that he had been nearby when the explosion occurred and rushed to the scene.
“I heard the sound of a big explosion and saw the blaze rising up to the sky, followed by smoke,” he said.
When he got to the scene, he said, he saw charred and unrecognizable victims, but he did recognize one: a man who works with him named Mohammed, who was severely burned. He rushed his colleague to the hospital, where he was being treated.