By Stephanie Nebehay

GENEVA (Reuters) - Syrians are caught between government bombardment of civilian areas and ruthless Islamist groups in a conflict increasingly driven by foreign powers and marked by the "spread of extremism", U.N. war crimes investigators said on Thursday.

Islamic State or ISIS forces, who control large parts of northern and eastern provinces, have expanded into the center and south, instilling terror and committing crimes against humanity, the investigators said.

The latest U.N. report documenting murders, rapes and abductions committed by all sides between January and July is based on 355 interviews as well as photographs, satellite imagery and medical records.

"A resonant cry for peace and accountability rings out," said the U.N. commission of inquiry led by Paulo Pinheiro, also urging the world to allow in more desperate Syrian refugees.

"The war is increasingly driven by international and regional powers, primarily in accordance with their respective geostrategic interests," the report said without naming names.

The Syrian army and Lebanese Hezbollah allies are fighting rebels in the four-year-old war and Damascus has received financial and military support from Iran. Syria has blamed some recent insurgent advances on support from Damascus's enemies Turkey, Qatar and Saudi Arabia.

"The competition among regional powers for influence has resulted...in an alarming exacerbation of the sectarian dimension, instigated by the intervention of foreign fighters and extremist clerics," the report said.

The government of President Bashar al-Assad has continued aerial bombing of residential areas of Aleppo, Deir al-Zor, Idlib, Damascus and Deraa, "leading to widespread civilian casualties". State helicopters have dropped barrel bombs, it added, calling for a halt to the use of illegal weapons.

"Investigations are continuing into the alleged use of chemical weapons, in the form of chlorine and/or phosgene gas, in Sarmin, Saraqeb, Qmenas and Binnish, as well as other towns and villages in Idlib in March and April," the report said, referring to allegations that barrel bombs containing poisonous gas were dropped in the northwest province.

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The investigators, who have interviewed 600 former prisoners since 2011, said: "Almost all have been victims and or witnesses of torture. Many have been present at the death of cellmates."

"The government is responsible for the deaths of detainees on a massive scale."

ISIS, whose Syrian stronghold is in Raqqa, has escalated attacks on Homs and Hassaka provinces, the report said. Its forces have attacked Kurdish communities, the most deadly being in June in Kobani where an estimated 250 civilians were killed.

"ISIS has committed murder, torture, rape, sexual slavery, sexual violence, forcible displacement and other inhumane acts as part of a widespread attack on the civilian population, amounting to crimes against humanity," it said.

These also constituted war crimes, along with ISIS using children in combat and attacking protected cultural objects.

Jabhat al-Nusra, linked to al-Qaeda, has led rebel gains in Idlib, "imposing its extremist ideology".

"The Commission continues to investigate reports of ISIS fighters throwing gay men off high buildings and their being beheaded by Jabhat al-Nusra," it said.

A U.S.-led airstrike on Beir Mihli in Aleppo in April caused more than 60 civilian casualties, while its assault on Dali Hasan in Aleppo in June killed a family including five children, the report said.

The military objective was not clear in either incident.

(Reporting by Stephanie Nebehay; Editing by Toby Chopra)