The three countries agreed a year ago to establish a "de-escalation zone" in the opposition-held Idlib province and surrounding region, which borders Turkey, but Syrian forces have since launched an offensive in the area.
He said that this is undermining efforts to reach a political solution to the war in that country.
Russian-backed regime forces launched an operation on the edge of Idlib province in the last days of 2017 and have retaken villages every day since.
"If the strikes on civilian centers continue, there is the possibility of an additional 400,000 civilians trying to make their way to the Turkish border".
The SOHR estimates that over the past two weeks, the Syrian army has managed to take more than 60 villages in Idlib province.
Idlib province, which borders Turkey, is nearly entirely controlled by anti-government forces that are dominated by a jihadist outfit known as Hayat Tahrir al-Sham (HTS) consisting mostly of fighters from a former Al-Qaeda affiliate.
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Israel insists that it supports neither of the parties to the Syrian conflict and is not involved in the talks to resolve the crisis in that country.
The Observatory said the attacks were carried out by an Islamist rebel faction that operates in rural Latakia province, southwest of Idlib, and which had targeted the air base a number of times since December 31.
But treatment now is becoming increasingly hard to find in Eastern Ghouta, and other areas where rebel groups are still trying to hold ground. It has remained in rebel hands despite subsequent factional fighting and relentless government bombardment, including chlorine and sarin gas attacks. Fighting and airstrikes in those areas violate a truce proposed by Russian Federation, which was accepted by Syrian President Bashar al-Assad in November.
The regime's chokehold on the enclave has led to a desperate humanitarian situation there, with the government repeatedly refusing aid into the area or to allow civilians to easily leave.
"I think the idea was not only to deposit residents of these "reconciled" areas in an opposition-held dumping ground, but to do it in the least hospitable one possible - one run by predatory jihadist factions, and that would be culturally alien to people from, for example, the Damascus countryside", he said.
Shelling and air strikes on Ghouta, which government forces have besieged for four years, killed at least 20 civilians, the Syrian Observatory for Human Rights said.
The monitor, which gathers its information from a network of sources inside Syria, added that it was not clear whether the attack was specifically targeting the group's base.