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 gathered 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.
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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."
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.
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