The officials said 65 other people were wounded in the attack, which took place on Thursday night at a gas station on a major highway near the city of Hilla, about 95 kilometers (60 miles) south of the Iraqi capital.
 
IS claimed the attack in a brief statement on its Aamaq media arm, saying it was a suicide truck bomb. Earlier, Iraqi officials, who spoke on condition of anonymity because they were not authorized to talk to the media, had put the death toll at 56.
 
The attack appears to have targeted a bus with Iranian pilgrims heading home after a major Shiite religious observance in the holy city of Karbala.
 
Iran's Deputy Foreign Minister Hassan Qashqavi was quoted by the semi-official Tasnim news agency on Thursday night as saying that 80 people were killed, including 40 Iranians. Conflicting death tolls are common in the aftermath of large attacks.
 
The attack was the deadliest by IS since a massive car bomb killed around 300 people in Baghdad in July. It also came a day after some dozen small-scale bombings in and around Baghdad killed 31 people and wounded more than a 100 _ a particularly bloody day even by the standards of the Iraqi capital, which has for more than a decade endured near-daily violence blamed on IS or its forerunner, al-Qaida in Iraq, and which mostly targeted members of Iraq's Shiite majority.
 
The gas station bombing underlined the continuing ability of IS to stage high-profile terror attacks even as a massive Iraqi military operation is underway to dislodge its fighters from its last major urban stronghold in Iraq, the northern city of Mosul. The offensive is aided by volunteer militiamen and the U.S.-led coalition, which has mostly been pounding IS targets in Mosul with airstrikes.
 
Moreover, Thursday's IS bombing took place in Iraq's Shiite hinterland south of Baghdad, a region that has largely been spared the near-daily violence that has for years engulfed the capital and Sunni regions.
 
Earlier this week, Prime Minister Haider al-Abadi visited Karbala, where he lavishly praised the country's security forces for protecting the pilgrims against attacks by IS.  
 
Extremist Sunni militants, including the Islamic State group, view Shiites as heretics and routinely target Iranian pilgrims who visit Iraq by the hundreds of thousands to pay homage to major Shiite shrines in Baghdad, and also the Shiite holy cities of Karbala and Najaf, south of Baghdad, and Samarra, to the north of the capital.
 
Shiite Iran is a major backer of the governments in Iraq and neighboring Syria in their fight against IS, providing military advisers and weapons.
 
``Let the vile Rafidha (Shiites) know that what awaits them in the near future, God willing, will be more painful and bitter and that the flames of the battles in Nineveh will reach them in Baghdad, Karbala and Najaf,'' IS said in its statement. Nineveh is the northern Iraqi province where Mosul is the capital.
 
The Shiite observance in Karbala, called the al-Arbaeen, in which the Iranian victims participated, was the 40th day after the death anniversary of a much revered 7th century religious figure, Imam Hussein, the grandson of the Prophet Muhammad. The ceremonies routinely attract hundreds of thousands of pilgrims, including many Iranians who travel overland into Iraq for the occasion.
 
Several members of Iranian civil defense staff who had been in Karbala to offer assistance to Iranian pilgrims rushed to the gas station following the bombing, helping their Iraqi counterparts collect the bodies of the victims and aiding the wounded. Iranian personnel typically accompany pilgrims on such trips to Iraq.
 
The Iraqi government's campaign to retake Mosul began last month, but stiff IS resistance and concerns over the safety of civilians who remain inside the city have slowed the Iraqi forces' progress.
 
Fighting continued in the eastern sector of Mosul on Friday, with Iraqi special forces seizing another neighborhood, Masaref, and advancing in the densely populated Zohour district, according to Brig.Gen. Haider Fadhil. The offensive to capture Zohour began earlier this week, but troops are facing stiff IS resistance, he added.
 
On the ground in Mosul, the sounds of automatic fire and the thud of mortar shells and artillery shook the city's eastern sector, east of the Tigris River on Friday morning. (AP)