The Islamic State extremist group and other Sunni insurgents seized control of the city of Fallujah, as well as part of the nearby city of Ramadi in Anbar province in early January. The militants then launched a massive blitz in June that has brought a huge swath of northern and western Iraq under their control.
In its report, the U.N. mission to Iraq said at least 5,576 civilians were killed and another 11,665 wounded in Iraq from Jan. 1 through the end of June. Another 1.2 million people have been driven from their homes by the violence, it said.
The pace of civilian deaths over the first six months marked a sharp increase over the previous year. In all of 2013, the U.N. reported just over 7,800 civilians killed, which was the highest annual death toll in years.
The fighting "has inflicted untold hardship and suffering on the civilian population with large-scaled killings, injuries, and destruction and damage of livelihoods and property," the U.N. said.
The report also documents human rights abuses by both sides of the conflict that may constitute crimes against humanity or war crimes.
The U.N. said the Islamic State group and its allies have committed "systematic and egregious violations" against civilians, including killings, sexual violence, kidnappings, destruction of property and attacks on places of religious worship.
It also documented violations by government forces, including summary executions of prisoners and detainees.
The U.N. urged all sides in the conflict to ensure the protection of civilians, and to respect international law and humanitarian law.
Also Friday, Iraqi President Jalal Talabani's office said the ailing leader would return Saturday from Germany, where he has been receiving medical treatment since 2012. Few details have been released about his condition.
Talabani is finishing up his second consecutive term as president, and is not eligible to run again.