Fierce clashes have been reported near Aleppo in northern Syria, as government forces try to surround rebels holed up in the strategic city.
Monitoring group the Syrian Observatory for Human Rights said about 120 fighters on both sides had been killed around the town of Ratyan on Friday.
As many as 20,000 refugees fleeing the fighting have spent the night at a border crossing with Turkey.
Turkey says it is prepared to help the refugees but the frontier is shut.
In the past few days, the Syrian army backed by Russian air power has made a series of gains in Aleppo province.
Syrian state TV said on Friday that pro-government forces had seized Ratyan, just north of Aleppo city.
Who is to blame for peace talks' suspension?
Displaced Syrians struggle to survive
The story of the conflict
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Waiting at the gates: Mark Lowen, BBC News, reports from Gaziantep
The thousands fleeing Aleppo have spent their first night in temporary shelter at the Turkish border but the crossing remains closed.
Turkey has welcomed more than two million Syrians since the start of the war and it is unclear whether it will accept another huge wave in one go.
Human rights groups are calling for the gates to open but at a time when the EU is trying to reduce the flow of refugees to Europe, there will be diplomatic pressure on Turkey to act with caution.
The Turkish government is using the new refugee exodus to take aim at Russia, whose air strikes on Aleppo have prompted thousands to flee. But there is no sign that Russia is pulling back and as opposition fighters retreat, the Assad regime is strengthened and thousands more civilians will flee, hoping for sanctuary in Turkey.
Rebel chief Hassan Haj Ali told Reuters the town had not yet fallen but there were "very heavy battles".
"The regime is now trying to expand the area it has taken control of," he added.
"Now the northern countryside [of Aleppo] is totally encircled and the humanitarian situation is very difficult."
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Media captionSyrians have fled Aleppo following air strikes, says Mark Lowen
Image copyrightAFPImage captionTurkey says it is prepared to provide shelter and food for those fleeing the fighting
Earlier in the week, the Syrian government claimed a major victory by breaking the rebel siege of two towns in Aleppo province, severing an opposition supply line from Turkey to Aleppo city.
"It feels like a siege of Aleppo is about to begin," said David Evans, a spokesman for aid group Mercy Corps.
Before the civil war, Aleppo was a key commercial centre and home to more than two million people. Since 2012 it has been divided into rebel- and government-held areas.
The UN Office for the Co-ordination of Humanitarian Affairs (OCHA) said up to 20,000 people fleeing the fighting in Aleppo province had gathered at the Bab al-Salam border crossing on the Turkish border.
The Syrian war and Aleppo
March 2011: Anti-government protests erupt across Syria, but Aleppo is initially untouched as a result of a state crackdown
February 2012: As the rebellion turns into a conflict, clashes between rebels and the government are reported with increasing frequency in Aleppo province
July 2012: The battle for Aleppo begins. Rebels make swift advances, but are unable to consolidate their gains and the city becomes divided
2013: The government begins bombarding rebel districts with barrel bombs, causing thousands of casualties
September 2015: Syria launches a fresh offensive in the wake of Russia's intervention in the conflict
February 2016: The government captures towns north of Aleppo, threatening to encircle the city
Aleppo profile
It said between 5,000 to 10,000 refugees had also fled to the nearby city of Azaz.
"Humanitarian organisations are responding to the needs of those displaced, but ongoing military conflict is making access to populations in need increasingly difficult," the UN's Linda Tom told AFP news agency.
In a televised speech, Turkish Prime Minister Ahmet Davutoglu said Turkey would not leave refugees "without food or shelter" but he would not say if they would be allowed in.
Nato has accused Russia of "undermining" Syrian peace efforts through its strikes, which it says are mainly aimed at opposition groups. Russia insists it only targets what it calls terrorists.
On Friday, Turkish President Recep Tayyip Erdogan accused Russia of being engaged in an "invasion" of Syria, saying it was trying to create a "boutique state" for ally President Bashar al-Assad.
Mr Erdogan said Russia and the Syrian government were together responsible for 400,000 deaths in Syria.
On Thursday Russia accused Turkey, key backer of Syria's opposition, of preparing a ground invasion, an accusation Mr Erdogan called "laughable".
Turkey and Russia have been embroiled in a row since Turkey shot down a Russian jet it accused of violating its airspace in November.
What is the Syria conflict?
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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, the Islamic State group, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and al-Nusra Front, as well as less numerous so-called "moderate" rebel groups, which are strongest in the north and east. These groups are also battling each other.
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 are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.
