Suicide bombers targeted two major cities in Iraq on Monday, striking security forces in the Sunni city of Fallujah and Shiite pilgrims in the holy city of Karbala in the country’s south, killing at least six people in each attack.

The bombings are an apparent attempt by the Islamic State group to strike back as government troops’ advance on their stronghold in the northern city of Mosul. The Sunni militant group claimed responsibility for both bombings.

In Fallujah, the attack was carried out by twin suicide car bombers in the city center — the first such incidents since Iraqi forces in late June declared it “fully liberated” of Islamic State militants after a monthlong operation aided by U.S.-led airstrikes.

Interior Ministry spokesman Brig. Gen Saad Maan said one of the Fallujah bombers killed two policemen and wounded 17 people, including nine civilians.

“They hit a security checkpoint,” he said, adding that traffic had been heavy in the area and around 10 vehicles including an ambulance had been damaged. The second bomber killed four people, including two policemen.

IS claimed responsibility for the attack, posting videos online of the bombers before the attack, their faces masked and brandishing assault rifles. Fallujah was the first Iraqi city to fall to IS, in January 2014. Since IS was driven out, families have begun returning to the city, 40 miles (65 kilometers) west of Baghdad.

Earlier in the day, a suicide bombing targeted the sacred Shiite city of Karbala, killing at least six civilians in an attack also claimed by IS.

According to Maan, the ministry spokesman, six suicide bombers tried to sneak into Karbala from its western outskirts but security forces detected and killed five of them. The sixth attacker broke into a house in the city’s Ayn al-Tamer neighborhood, where he detonated his suicide vest, killing six people and wounding another six, Maan said.

The attack came as hundreds of thousands of faithful Shiites have been walking to Karbala to commemorate the 7th century death of Imam Hussein, Prophet Muhammad’s grandson and an iconic Shiite martyr. The city, where Imam Hussein and his brother are buried, is located some 90 kilometers (55 miles) south of Baghdad. On Sunday, pilgrims are expected to converge on the city for the climax of a 40-day mourning period, known as al-Arbaeen.

In an online statement, the Islamic State group said it targeted “polytheists ... in filthy Karbala.” It put the number of the suicide bombers at five, saying they clashed for hours with the security forces and set off their explosives consecutively when they ran out ammunition.

The militants often exaggerate their claims. The Associated Press could not verify the authenticity of the statement, but it was posted on a militant website commonly used by the extremist group.

Like other Sunni extremist groups, IS considers the Shiites heretics and has frequently targeted Shiite civilians and places of worship.

“We are walking all these long distances, we are not afraid of terrorists, we are not afraid of Daesh,” said 56-year-old pilgrim Sattar Hussein, using the Arabic acronym for IS. “We are not afraid of anyone who defames Islam. And especially those who are targeting the al-Arbaeen.”

Wearing a black traditional Arab dress, Hussein was among hundreds of pilgrims leaving Baghdad on foot to head to Karbala under tight security measures. Some of the marchers were carrying religious flags, including some with a portrait depicting Imam Hussein, as security forces set up check points to search them.

In southern Iraq’s Shiite heartland, Karbala is far from Mosul, where a massive Iraqi military operation — launched last month and backed by U.S.-led coalition airstrikes and paramilitary militias — is underway to free the city, Iraq’s second-largest, from IS.

A U.N. spokesman said Monday that more than 54,000 people have fled their homes as a result of the Iraqi military operation to retake Mosul from IS militants. Farhan Haq said that number represented an increase of 6,600 people since four days ago.

About three quarters of the displaced people are being shelter in camps set up by the U.N. and its humanitarian partners and one quarter are been housed in host communities, Haq said.

He added that the World Food Program has provided food assistance to more than 100,000 people fleeing the conflict, including a distribution to 25,000 people on Sunday in Gogjali, the first neighborhood retaken inside Mosul.

On Sunday, a wave of attacks in and around Baghdad killed at least 23 people and wounded 70 others. Many of the assaults targeted Shiite pilgrims walking toward Karbala.