At least 61 medical facilities have been hit in the area since April, a spokesperson for the UN High Commissioner for Human Rights said last week. But an internal document seen by dpa lists only seven sites that the inquiry is focused on, including a school, a refugee camp and hospitals.
According to a report in the New York Times, diplomats say Russia was pressuring Guterres not to publish the conclusions of the investigation.
Since April, Syrian forces, bolstered by Russian air power, have been waging a massive campaign against rebels in the north-western province of Idlib, the last opposition stronghold in the war-torn country.
On Thursday, Russia on Thursday did not deny allegations that it is pressuring Guterres to keep the findings of the probe under wraps.
When asked about the allegations during a Security Council meeting, Russia's representative said the Board of Inquiries was an "internal instrument."
"The report will be presented to the secretary general. It is for him to decide what to do with it," Dmitry Polyanskiy said through a translator.
Farhan Haq, a spokesman for Guterres, told dpa the UN was "still considering what information could be made public, although board of inquiry reports are internal documents."
Haq said the inquiry could be completed before the end of the year.
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