Tens of thousands of Syrians have fled a government offensive on rebel-held areas south of the city of Aleppo in the past three days, an activist says.
Dr Zaidoun al-Zoabi, head of the Union of Syrian Medical Relief Organisations, told the BBC that several villages he had visited were empty.
He saw thousands of people on the move, with no shelter or medical support.
The government offensive is the latest of at least four launched with Russian air support in the past two weeks.
Aside from the Aleppo countryside, they are taking place in mostly rural areas north of the cities of Homs and Hama, and in the north of the coastal province of Latakia.
Rebel fighters - not including jihadist militants from Islamic State (IS) - had penetrated most of these strategically important areas earlier this year.
Their gains prompted Russia to launch an air campaign to bolster President Bashar al-Assad in September, and also reportedly led Iran to deploy hundreds of combat troops. Tehran has previously only acknowledged sending military advisers.
'Sky filled with jets'
Dr Zoabi told the BBC's Newsday programme that he had seen at least 70,000 people on the move in the countryside south of Aleppo.
"We saw only people who do not have even tents, any shelter, whatever. People were asking for some food, sandwiches even," he said. "There is no medical support."
"The shelling is so fierce. The sky was filled with jet fighters, with helicopters, and people are terribly scared. They are scared to death."
The Syrian Observatory for Human Rights said government forces, backed by Syrian and Iranian militiamen and fighters from Lebanon's Hezbollah movement, had taken control of three hills near the town of Khan Tuman.
Rebels from the Western-backed Free Syrian Army meanwhile told the Reuters news agency that they had received new supplies of US-made anti-tank missiles since the assault began, though not enough to repel it.
The government's forces appear to be pushing towards the motorway that links Aleppo with Latakia and Hama provinces, rebels say.
Aleppo is about 50km (30 miles) from the border with Turkey. The city, once Syria's commercial and industrial hub, has been divided in roughly two since 2012, with the government controlling the western half and rebel factions holding the east.
On Sunday, Turkey's Prime Minister Ahmet Davutoglu warned European leaders that the offensive south of Aleppo, as well as attacks in the area by Islamic State militants, could "cause another wave of refugees".
More than 11 million Syrians have been driven from their homes since 2011. More than 4.18 million have fled abroad - 2.07 million of them to Turkey - with growing numbers now making the dangerous journey to Europe.
Syrian government forces were also battling IS militants on the eastern edge of Aleppo on Monday, as well as in the eastern city of Deir al-Zour and eastern parts of Hama province.
In all those areas, Russia says its air strikes have destroyed some important IS installations, including command posts, underground bunkers, and arms and ammunition depots, the BBC's Jim Muir reports from Beirut.
Syria's civil war
Why is there a war in Syria?
Anti-government protests developed into a civil war that four years on has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
What's the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad - including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran have pro-Assad forces on the ground, while a Western-led coalition and Russia are carrying out air strikes.