At least 70,000 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.
Dr Zoabi told the BBC’s Newsday programme that he had seen thousands of 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.
Image copyright Reuters Image caption Much of Aleppo, once Syria’s commercial and industrial hub, has been devastated by fighting
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”.
Image copyright Reuters Image caption Turkey’s prime minister says the Aleppo offensive 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.
The helicopter “hit the cable of the security balloon over the base. It seemed like the cable was turned around the helicopter rotor”, he said. Najibullah said the helicopter went down and then black smoke rose from the area.
A second helicopter circled around the area three or four times and then landed at a nearby airport, he said.
Tribus would only say that an “incident” involving a NATO aircraft and an observational balloon had taken place “in the vicinity of the Resolute Support base” in central Kabul.
The monitoring balloon was severed from its mooring in the incident, he said, without providing further details.