The UN Secretary-General, meanwhile, has raised concerns about the "alarming military escalation" in the country
The UN human rights chief has called for urgent international action in Syria after a week he described as "one of the bloodiest periods of the entire conflict".
According to the UN, at least 277 civilians are reported to have been killed in the country between February 4th and 9th - with that number including 230 people killed in airstrikes by the Syrian government and its allies.
More than 800 civilians are believed to have been injured in the same period.
There has been a particular increase in military activity in the opposition-held Eastern Ghouta and Idlib regions.
Saturday saw a further escalation of tensions, when an Israeli fighter jet crashed after it was apparently shot down during airstrikes against "Iranian targets" in Syria.
Israeli military said it had conducted strikes against 12 targets in Syria when its planes came under anti-air fire, with one jet ultimately crashing in the Jezreel Valley in northern Israel. Both pilots survived the crash.
Israeli officials claimed they had launched the strikes after an Iranian drone entered Israeli airspace.
Iran, meanwhile, accused Israel of "lies" and said Syria had the right to "legitimate self-defence" in response to strikes.
In a statement, UN Secretary-General António Guterres called on the parties involved in the ongoing conflict to move toward a political solution, which he says is the "only way to end the violence and the terrible suffering of the Syrian people".
A spokesperson for the UN chief said: "The Secretary-General is following closely the alarming military escalation throughout Syria and the dangerous spillover across its borders.
"The Secretary-General stresses once again that all concerned in Syria and the region have a responsibility and must abide by international law and relevant Security Council resolutions. He calls on all to work for an immediate and unconditional de-escalation of violence and exercise restraint."
UN High Commissioner for Human Rights Zeid Ra’ad Al Hussein, meanwhile, said urgent international action was needed after a 'week of soaring violence and bloodshed'.
He stated: “The past week has been one of the bloodiest periods of the entire conflict, with wave after wave of deadly airstrikes leading to civilian casualties in areas of Eastern Ghouta and Idlib.
“The no-holds-barred nature of this assault is evidenced by reports that at least nine medical facilities, six of them in Idlib and three in Eastern Ghouta, were hit by airstrikes."
He added: "Even by Syria’s atrocious standards, these are exceptionally deplorable developments - and a cruel irony given that both have been declared 'de-escalation areas'."
He noted that the failure to end the war "marks an epic failure of global diplomacy".
The conflict in Syria has seen more than 300,000 people killed and millions more displaced since it began in early 2011.