(Adds dropped words "this year" in 3rd paragraph)

* Mass executions, abductions by IS deemed war crimes

* UN rights chief denounces "staggering array" of abuses

* Iraqi air strikes cause "significant civilian deaths"

By Stephanie Nebehay

GENEVA, Oct 2 (Reuters) - Islamic State insurgents in Iraq have carried out mass executions, abducted women and girls as sex slaves, and used child soldiers in what may amount to systematic war crimes that demand prosecution, the United Nations said on Thursday.

In a report based on 500 interviews with witnesses, also said Iraqi government air strikes on the Sunni Muslim militants had caused "significant civilian deaths" by hitting villages, a school and hospitals in violation of international law.

At least 9,347 civilians had been killed and 17,386 wounded so far this year through September, well over half of them since the Islamist insurgents also known as ISIL and ISIS began seizing large parts of northern Iraq in early June, the report said.

"The array of violations and abuses perpetrated by ISIL and associated armed groups is staggering, and many of their acts may amount to war crimes or crimes against humanity," said U.N. High Commissioner for Human Rights Zeid Ra'ad al Hussein.

In a statement, he called again for the Baghdad government to join the International Criminal Court, saying the Hague court was set up to prosecute such massive abuses and direct targeting of civilians on the basis of their religious or ethnic group.

Islamist forces have committed gross human rights violations and violence of an "increasing sectarian nature" against groups including Christians, Yazidis and Shi'ite Muslims in a widening conflict that has forced 1.8 million Iraqis to flee their homes, according to the 29-page report by the U.N. Human Rights Office and the U.N. Assistance Mission for Iraq (UNAMI).

"These include attacks directly targeting civilians and civilian infrastructure, executions and other targeted killings of civilians, abductions, rape and other forms of sexual and physical violence perpetrated against women and children, forced recruitment of children, destruction or desecration of places of religious or cultural significance, wanton destruction and looting of property, and denial of fundamental freedoms."

FEMALE "SEX SLAVES"

In a single massacre on June 12, the report said, about 1,500 Iraqi soldiers and security officers from the former U.S. Camp Speicher military base in Salahuddin province were captured and killed by Islamic State fighters.

However, the bodies have not been exhumed and the precise toll is not known. No one disputes that Iraqi military recruits were led off the base near Tikrit unarmed and then machinegunned in their hundreds into mass graves by Islamic State, whose fighters boasted of the killings on the Internet.

Women have been treated particularly harshly, the report said: "ISIL (has) attacked and killed female doctors, lawyers, among other professionals."

In August, it said, ISIL took 450-500 women and girls to the Tal Afar citadel in Iraq's Nineveh region where "150 unmarried girls and women, predominantly from the Yazidi and Christian communities, were reportedly transported to Syria, either to be given to ISIL fighters as a reward or to be sold as sex slaves."

Islamic State pushed on with its assault on a Syrian border town on Thursday despite coalition air strikes meant to weaken them, sending thousands more Kurdish refugees into Turkey and dragging Ankara deeper into the conflict.

Islamic State and allied groups have attacked and destroyed places of religious and cultural significance in Iraq that do not conform to its "takfiri" doctrine, the U.N. report said, referring to the beliefs of Sunni militants who justify their violence by branding others as apostates.

But the report also voiced deep concern at violations committed by the Baghdad government and allied fighters, including air strikes and shelling that may not have distinguished between military targets and civilian areas. (additional reporting by Ned Parker in Baghdad; Editing by Louise Ireland)