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Guterres also expressed hope that a truce agreed in the Idlib region by the Syrian government on Thursday would hold, with the ceasefire contingent on implementation of a Turkish-Russian buffer zone deal.

"I hope that the present dramatic situation that is taking place will end," he told reporters.

Heavy bombardment by regime and Russian forces on Idlib, Syria's last jihadist-run stronghold, and other parts of the northwest has killed over 780 civilians since the end of April, according to the British-based Syrian Observatory for Human Rights. 

More than 400,000 people have been displaced by the flare-up over the same period, the United Nation says.

Idlib province along with northern Hama and parts of Aleppo and Latakia are under the control of Hayat Tahrir al-Sham, a jihadist group led by Syria's former al-Qaeda affiliate.

Read more: Syria Weekly: Assad calls a truce, but Idlib's healthcare is ravaged by months of bombing

The region was supposed to have been spared from a threatened government offensive last year by a deal struck by Ankara and Moscow in the Russian resort of Sochi in September.

But the agreement has faltered and Syrian forces have launched an intensified bombardment of the region with Russian support since late April.

According to the Observatory, some 790 civilians have been killed in the past three months along with 1,000 rebels and around 900 pro-government fighters.

The UN says that more than 400,000 people have been displaced by the fighting.

"The investigation will cover destruction of, or damage to facilities on the deconfliction list and UN-supported facilities in the area," a spokesman for Guterres said in a statement announcing the inquiry.

"The Secretary-General urges all parties concerned to cooperate with the board once it has been established," he added.

However, Russia's Deputy UN Ambassador Dmitry Polyanskiy slammed the probe, describing allegations that its or Syrian regime forces were behind the incidents as "fake news."

"We doubt very much that this is for the sake of an investigation. This is for the sake of blaming Syria and Russia for things that we do not do," he told reporters.

The war in Syria has killed more than 370,000 people and displaced millions since it started in 2011 with a brutal crackdown on anti-government protests. 

Agencies contributed to this report.

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