* U.N. report says both sides may have committed war crimes
* Religious sites plundered during Islamic State assault
* Maliki, facing opposition, wants to stay on as premier
By Dominic Evans and Maggie Fick
BAGHDAD, July 18 (Reuters) - The United Nations accused Islamic State fighters in Iraq of executions, rape and forced recruitment of children during a campaign to seize much of northern Iraq, part of a conflict it said has killed almost 5,600 civilians this year.
In a report, the U.N. focused on a range of violations committed against civilians, particularly by the Islamic State, though it also said Iraqi forces and allied fighters had not taken precautions to protect civilians from violence.
"(This)...may also amount to war crimes," it said in its report into months of unrest which culminated in advances by Sunni militants led by the al Qaeda offshoot Islamic State, formerly known as ISIL, across the north of the country.
"ISIL and associated armed groups have also continued to... perpetrate targeted assassinations (of) community, political, and religious leaders, government employees, education professionals, health workers... sexual assault, rape and other forms of sexual violence against women and girls, forced recruitment of children, kidnappings, executions, robberies."
The report, the U.N.'s most comprehensive review of the impact of months of unrest, also accused the Islamic State of wanton destruction and plundering of places of worship and of cultural or historical significance.
"Every day we receive accounts of a terrible litany of human rights violations being committed in Iraq against ordinary Iraqi children, women and men, who have been deprived of their security, their livelihoods, their homes, education, healthcare and other basic services," U.N. High Commissioner for Human Rights Navi Pillay said.
Iraq's Interior Ministry, citing testimony from a survivor, said this week that an investigation had revealed the Islamic State took 510 Shi'ite prisoners from a Mosul prison to nearby farmland and executed all but 17 who managed to flee.
The report also detailed violations committed by government forces and affiliated groups, citing "summary executions/extrajudicial killings of prisoners and detainees", which it said may constitute a war crime.
MALIKI SAYS STAYING PUT
Of the 2,400 people killed in June, 1,531 were civilians, the U.N. said earlier this month.
The report called on the government to investigate serious violations and to hold the perpetrators to account. But the capacity of the Shi'ite-led caretaker government to do so in the face of a Sunni uprising that threatens to fracture the country on sectarian and ethnic lines may be limited.
Iraqi politicians have yet to complete the formation of a new government more than three months after parliamentary elections. Prime Minister Nuri al-Maliki faces pressure from Sunnis, Kurds and some Shi'ites to step aside after two terms in office in which his critics say he marginalised opponents.
In an interview published on Friday, Maliki, whose Shi'ite State of Law bloc won the most seats in parliament, reiterated his determination to form the next government.
"In free and democratic elections, we won the most votes and the constitution says that the leader of the strongest faction forms the government, so I want to run again for the post of prime minister in parliament," Maliki said in an interview with German mass-selling daily Bild.
He said some groups were trying to bypass the election result: "But we insist that the opinion and decision of the people be respected, and we won't make any compromises at the expense of the people and the legitimacy of the state."
As well as a new prime minister, parliament must agree on a likely replacement for President Jalal Talabani, who has been in Germany for treatment since suffering a stroke in December.
Talabani's office said he will fly back to Iraq on Saturday.
Although the presidency is a largely symbolic role, Talabani was widely seen as a unifying figure, both within Iraq and his own Patriotic Union of Kurdistan (PUK) party, which has struggled to contain internal divisions without him at the helm.
This week the PUK's Najmaldin Karim, who is also governor of Kirkuk, submitted his candidacy for president without the blessing of party leaders, sources said, challenging Barham Saleh, who had been touted as frontrunner for the position.
In the first, silent footage to be broadcast of him earlier this year, Talabani appeared much weakened, suggesting he is unlikely to be able to resume an active role in politics.
Parliament is due to meet next Wednesday for the next session of talks on choosing a political leadership to tackle the conflict, which has displaced more than 1 million people this year, according to the United Nations.
Uprooted by the violence, the displaced live in "cruel and difficult" conditions, the country's top Shi'ite cleric said in Friday sermon, criticising Iraqi and international agencies for ignoring their plight.
"The institutions concerned with this are still not meeting the scale of the hardships and suffering, despite the promises that we heard of help," Grand Ayatollah Ali Sistani said in a sermon delivered by an aide the Shi'ite holy city of Karbala.
Security sources and witnesses said fighting continued on Friday in several towns north of Baghdad. Near the military base at Muqdadiya, 50 miles (80 km) north of the capital, militants blew up a bridge to prevent Kurdish Shi'ite tribes bringing reinforcements to confront them, they said. (Additional reporting by Isabel Coles in Arbil, Raheem Salmana and Isra' al-Rube'i in Baghdad and Michelle Martin in Berlin; Editing by Hugh Lawson)