Syrian rebel forces arrived at the gates of the key city of Hama on Tuesday, as their fighting with the military sparked "a large wave of displacement", a war monitor said.
The Islamist-led rebels were advancing on Syria's fourth-largest city, buoyed by their lightning capture of swathes of the north in an offensive that ended four years of relative calm.
The sudden flare-up in the more than decade-old civil war in Syria drew appeals for de-escalation from across the international community.
The Syrian Observatory for Human Rights on Tuesday evening said the rebels were "now at the gates of Hama city" and that fighters had shelled some neighbourhoods.
The Britain-based Observatory, which has a network of sources in Syria, said Hama was witnessing "a large wave of displacement" due to fighting around the city.
It also reported people being displaced from villages in parts of Hama province's north and west.
AFP images showed people fleeing the town of Suran, between Aleppo and Hama, many of them carrying whatever they could take aboard their vehicles.
"Large military reinforcements have arrived to the city of Hama to bolster the forces on the front lines and confront any attempted attack," state news agency SANA reported, citing a Syrian military source.
A statement from Syria's army command said its forces were striking "terrorist organisations" in north Hama and Idlib provinces, with Russian air support.
Russia is a key ally of President Bashar al-Assad. Its 2015 intervention in the Syrian civil war turned the tide in his government's favour but since 2022 the Ukraine war has tied down much of its military resources.
Hama was a bastion of opposition to the Assad government early in the civil war.
Its capture by the rebels would "pose a threat to the regime's popular base", Observatory director Rami Abdel Rahman said.
The countryside west of the city is home to many Alawites, followers of the same offshoot of Shiite Islam as the president and his security chiefs.
An AFP journalist in the northern Hama countryside saw dozens of Syrian army tanks and military vehicles abandoned by the side of the road leading to Hama.
"We want to advance on Hama after combing" towns that have been captured, a rebel fighter who identified himself as Abu al-Huda al-Sourani told AFP.
AFPTV footage showed rebel fighters clashing with the Syrian military in Halfaya, about 20 kilometres (12 miles) northwest of Hama.
The United Nations says nearly 50,000 people have been displaced by the fighting since it began last Wednesday.
At least 602 people have been killed, mostly combatants but also including 104 civilians, according to the Observatory.
The exodus of civilians just as winter takes hold has triggered international concern.
UN chief Antonio Guterres said he was "alarmed" by the violence and called for an immediate halt.
The European Union and the United States called on all sides to de-escalate.
Assad is no longer the pariah in the Arab world that he was at the height of the civil war.
At a summit in Cairo last year, Arab leaders agreed to reinstate Syria's membership of the Arab League, marking the start of a slow rehabilitation.
Turkish ally Qatar has been the main exception, refusing to normalise relations with Assad's government.
Its foreign ministry spokesperson Majed Al-Ansari denied any military role in the rebel offensive but said a negotiated settlement between the warring parties was the only way to end the conflict.
One anxious resident of Syria's second city Aleppo, who declined to be identified, spoke of panic as the rebels overran it on Friday and Saturday.
"There were terrible traffic jams," he said.
But others remained trapped inside the rebel-controlled city.
HTS has its origins in Al-Qaeda's former Syria branch and has faced accusations of human rights abuses including torture.
Nazih Yristian, 60, who lives in Aleppo's Armenian neighbourhood, said he and his wife had tried to flee but the main road out had been cut. Since then, the couple have cloistered themselves at home, he said.
"No one attacked us so far, but we want to leave until things calm down. We have been displaced a lot and we will be displaced again."
The Norwegian Refugee Council warned that the rebel offensive "threatens to drag the country back into the darkest days of this near 14-year conflict".
"In Aleppo, NRC teams report food shortages as bakeries and shops shut down. Damage to water networks has also reduced domestic water supplies," its Middle East and North Africa director Angelita Caredda said.
Russian President Vladimir Putin and his Iranian counterpart Masoud Pezeshkian pledged "unconditional support" for their ally, according to the Kremlin.
Putin later urged a "speedy" end to the offensive during a phone call with his Turkish counterpart Recep Tayyip Erdogan.
Turkish forces and their proxies have controlled swathes of territory in northern Syria since 2016.
Iran's Foreign Minister Abbas Araghchi meanwhile said: "If the Syrian government asks us to send forces to Syria, we will study their request."
Neighbouring Iraq too has expressed support, and on Tuesday a pro-Iran group within the security forces called on the government to go further and send combat troops.
A spokesman for Kataeb Hezbollah, part of the Iran-backed "axis of resistance", said the group had not yet decided to deploy its own fighters but urged Baghdad to act.