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.
RELATED: Iraqi Yazidis caught in Islamic State advance
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.
Dam and Christian town seized
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.
|Aziz Emanuel, who is in charge of the Iraq Church Development Project, talks to Al Jazeera from Erbil
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.
In a statement on Thursday, the UN said that it was gravely concerned by the serious deterioration in the security and humanitarian situation in northern Iraq.
"[The] Islamic State has overrun areas in the Ninewa plains and Shirkhan districts of Ninewa, with fighting continuing between [the Islamic State] and the Kurdish forces along border areas of the Kurdistan region in the vicinity of Makhmour," the UN said.
According to the statement, t he Deputy Special Representative of the UN Secretary-General for Iraq, Gyorgy Busztin the, called on the government of Iraq and the Kurdish government of northern Iraq to coordinate a security and humanitarian response to the current crisis.
Busztin the said governments should do their utmost to ensure the availability of resources to support the humanitarian needs of those civilians displaced or affected by the violence.