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 Assad's government in a lightning offensive.
The European Union also called on "all sides to de-escalate," while U.N. 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 U.N. Office for the Coordination of Humanitarian Affairs reported Monday.
Speaking to reporters, 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 airstrikes 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.
Panic in Aleppo
Aleppo is home to 2 million people and saw fierce fighting earlier in the war.
The Islamist Hayat Tahrir al-Sham and its allies took the city over the weekend, except for neighborhoods controlled by Kurdish forces, according to the Syrian Observatory for Human Rights.
They also seized Aleppo International Airport.
HTS, led by al-Qaida's former Syria branch, has faced accusations of human rights abuses including torturing detainees.
Abu Sufyan, a rebel commander, told AFP: "God willing, we will continue, go into Damascus, and liberate the rest of Syria."
One Aleppo resident spoke of panic.
"There were terrible traffic jams — it took people 13 to 15 hours to reach Homs" in central Syria, which is under government control. Normally, he said, it would take a couple of hours.
AFPTV footage showed rebels patrolling the streets, some burning a Syrian flag and others holding the flag of the revolution.
On Monday, Assad branded the rebel offensive led by HTS an attempt to redraw the regional map in line with U.S. interests.
His comments came in a call with Iran's Pezeshkian, who in turn pledged continued support and said Iran hoped "Syria will pass through this stage with success and victory."
Deadly Attacks
On Monday, Syrian and Russian air raids on several areas of Idlib province in the northwest killed 11 civilians including five children, the Observatory said.
"The strikes targeted ... families living on the edge of a displacement camp," said Hussein Ahmed Khudur, a 45-year-old teacher who sought refuge at the camp from fighting in Aleppo province.
Other strikes in Aleppo killed four civilians, two of them children, the Observatory said, adding that air raids also targeted a Christian-majority neighborhood.
AFPTV footage showed rebels pushing into Hama province in central Syria.
Syria's defense ministry said troops were clashing with "terrorist organisations" in the northern Hama countryside.
Islamist-led rebels killed six civilians on Monday in a rocket attack on the government-held city of Hama, the Observatory said.
'Instability'
Aron Lund of the Century International think tank said a major question hangs over possible Turkish involvement.
"I have a hard time imagining that Turkey-backed groups could launch a major rebel offensive out of Turkey-held areas without Turkey being very significantly involved," he said.
"But I also don't think Turkey necessarily wanted the rebels to get this far."
Turkish President Recep Tayyip Erdogan called for an end to the "instability" in Syria and an agreement to stop the civil war.
On a visit to Ankara, Iranian Foreign Minister Abbas Araghchi said it was crucial "to protect the achievements" of the so-called Astana process to end Syria's civil war, which involves Turkey, Russia, and Iran.
Several hours later, he said the respective foreign ministers would meet on the matter next weekend in Qatar.
"We will try to activate this process again," he said.
'Limited Utility'
Millions of Syrians have been displaced, with about 5.5 million now in neighboring countries.
Most of those involved in the initial anti-Assad protests are either dead or in jail or exile.
Russia is at war in Ukraine, and Iran's militant allies Hezbollah and Hamas have been massively weakened by more than a year of conflict with Israel.
Lebanon's Hezbollah played a key role in backing government forces, particularly around Aleppo, but withdrew from several positions to focus on fighting Israel.
HTS and its allies launched their offensive on Wednesday, the day a Lebanon ceasefire began.
The violence in Syria has killed 514 people, mostly combatants but also including 92 civilians, according to the Observatory.