* Heavy air strikes have killed hundreds -UN aid coordinator
* September deadliest month this year in Syria -UN
* Spike in military operations even in de-escalation areas
* Civilian deaths, hospital attacks "outrageous, unacceptable" -ICRC (Adds quotes from interview with senior ICRC official)
By Stephanie Nebehay
GENEVA, Oct 5 (Reuters) - Syria is in the throes of its worst fighting since the battle for eastern Aleppo last year, with heavy air strikes causing hundreds of civilian casualties, aid agencies said on Thursday.
Hospitals, schools and people fleeing violence have been "targeted by direct air strikes" that may amount to war crimes, the United Nations said, without apportioning blame.
Russia and a U.S.-led coalition are carrying out separate air strikes in Syria ostensibly aimed at defeating Islamic State militants.
"September was the deadliest month of 2017 for civilians with daily reports of attacks on residential areas resulting in hundreds of conflict-related deaths and injuries," U.N. regional humanitarian coordinator Panos Moumtzis said in a statement.
Air strikes killed dozens this week in Raqqa, where 8,000 people remain trapped, and at least 149 people, mostly women and children, in residential areas of rebel-dominated Idlib province in Syria's northwest in the last 12 days of September, he said.
Explosions in Damascus killed 20 people and civilian casualties were also reported in rural areas around the Syrian capital and in Hama, Aleppo and Deir al-Zor, Moumtzis said, again without saying who was responsible.
The International Committee of the Red Cross (ICRC) said in a statement that up to 10 hospitals were reported to have been damaged in the past 10 days.
"We have seen a number of hospitals being targeted, we got very worrying reports and converging reports about hospitals, schools, civilian infrastructure being targeted," Robert Mardini, ICRC regional director for the Middle East, told Reuters in an interview in his Geneva office.
"And of course this is outrageous, unacceptable," he said, adding that damage to Sham hospital in Idlib alone had cut off half a million people from access to health care.
The ICRC voiced alarm that violence was occurring in many of the "de-escalation" areas including Idlib, rural Hama and eastern Ghouta. "Taken together, these are the worst levels of violence since the battle for Aleppo in 2016," it said.
"Hundreds of civilians killed is certainly a very conservative figure," Mardini said. "We see a very strong correlation between the escalation and the intensification in the fighting on one hand and the human cost of this conflict, the attacks on health facilities and civilian infrastructure."
Syria's six-year-old civil war pits President Bashar al-Assad's government supported by Russia and Iran against a myriad number of rebel factions, some Western-backed, and Islamist militant groups.
Military jets believed to be Russian killed at least 60 civilians trying to flee heavy fighting in the oil-rich Deir al-Zor province when their small boats were targeted as they sought to cross the Euphrates River, opposition activists, former residents and a war monitor said late on Wednesday.
The U.S.-led coalition fighting Islamic State in Syria has carried out air strikes that have also caused civilian casualties, which it says it goes to great lengths to avoid.
"The point here is not to point fingers," Mardini said, declining to identify suspected perpetrators of deadly strikes.
"What should be non-negotiable is the respect for the laws of war, everywhere in Syria. This is absolutely critical today."
(Additional reporting by Tom Miles; editing by Mark Heinrich)
Our Standards: The Thomson Reuters Trust Principles.