Bomb attacks in the Iraqi capital of Baghdad and the Kurdish-controlled northern city of Kirkuk killed dozens of people, police and medical sources have said.
Suicide bomb attacks killed at least 35 people in Kadhimiya, a Shia area in northern Baghdad, and two other car bomb attacks killed nine people in Kirkuk, police sources said.
The explosions near a Shia mosque in Kirkuk holding displaced people came after an offensive launched by Islamic State fighters who have routed Kurdish forces in the north, causing thousands of people to flee.
The armed group, which is also active in Syria, has previously claimed responsibility for similar bombings.
Islamic State fighters have been advancing in various parts of the country, fighting against troops from the Kurdish and central governments.
The group sees Iraq's Shias and minorities such as Christians and Yazidis, a Kurdish ethno-religious community, as infidels.
Citing the risk of "genocide" in Iraq, US President Barack Obama on Thursday authorised the use of US air strikes in Iraq.
"We can act, carefully and responsibly to prevent a potential act of genocide," he said, referring to the Yazidis, besieged by fighters from the Islamic State.
Meanwhile, the group reportedly seized Iraq's largest dam, the country's largest Christian town and surrounding areas.
Qaraqosh, which has a population of about 50,000, lies between Mosul, the armed group's main hub in Iraq, and Erbil, the Kurdish region's capital in the north of the country
Thousands of panicked residents fled the city after its capture by the Islamic State, officials and witnesses told the AFP news agency.
The Islamic State also successfully stormed the Mosul Dam after a week of attempts and forced Kurdish peshmerga forces to withdraw from the area, residents living near the dam told the AP news agency.
The development placed the Sunni armed group in control of power and water resources as well as access to the river that runs through the heart of Baghdad.
The Islamic State group posted a statement online on Thursday, saying they had taken control of the dam and vowed to continue "the march in all directions'".
The statement could not be independently verified, but it was posted on a site frequently used by the group.
