BEIRUT - The United States called Monday for de-escalation in Syria, where an Islamist-led rebel alliance has wrested swathes of territory from the control of President Bashar al-Assad's government in a lightning offensive.
The European Union also called on "all sides to de-escalate", while UN Secretary-General Antonio Guterres said he was "alarmed" by the violence and called for an immediate halt to the fighting.
Syria has been at war since Assad cracked down on democracy protests in 2011. The conflict has since drawn in foreign powers and jihadists, and left 500,000 people dead.
The war had been mostly dormant with Assad back in control of much of the country, until last week when the Islamist-led rebel alliance began its offensive.
The attack has seen swathes of Syria fall to rebel control, including second city Aleppo for the first time since the start of the civil war.
Nearly 50,000 people have also been freshly displaced as of the end of November, the UN Office for the Coordination of Humanitarian Affairs reported Monday.
Speaking to reporters, US State Department spokesman Matthew Miller said: "We want to see all countries use their influence -- use their leverage -- to push for de-escalation, protection of civilians and ultimately, a political process forward."
In a statement issued by EU foreign affairs spokesman Anouar El Anouni, the European Union also called for de-escalation and the protection of civilians, while also condemning Assad backer Russia for conducting air strikes in his support.
Russia first intervened directly in Syria's war in 2015 with strikes on rebel-held areas.
Its help, along with that of Iran and Lebanese militant group Hezbollah, was instrumental in propping up Assad's rule.
On Monday, President Vladimir Putin and Iranian President Masoud Pezeshkian pledged "unconditional support" for their ally, according to the Kremlin.