Thousands of Syrian soldiers are being moved from the border with Turkey to join fierce fighting in the city of Aleppo, activists have claimed.
Rebels said their fighters had attacked columns of troops as they abandoned their posts in Jabal al-Zawiya area.
Aleppo, the country's second city, has seen fierce clashes all week, with the government deploying fighter jets and helicopters to beat back the rebels.
Government forces have largely repelled a rebel assault on Damascus.
Until recently, the two main cities had been relatively free of the violence that has wracked other parts of the country.
But a sustained assault by rebels earlier this month on the capital saw a dramatic upsurge in violence there.
Although government forces appear to have largely retaken control of Damascus, activists still say military helicopters are attacking some parts of the city.
The focus of the fighting appears to have moved hundreds of miles north to Aleppo, a northern city regarded as the country's commercial hub.
The BBC's Ian Pannell, near Aleppo, says on Tuesday fighter jets strafed parts of the city, and civilians and rebel fighters had been killed in fighting.
It was thought to be the first time that fighter planes had been deployed in such a way since anti-regime protests began in March 2011.
Early on Wednesday opposition activists said large numbers of troops began withdrawing from the Turkish border.
An activist called Abdelrahman Bakran told Reuters news agency that rebels had attacked the rear of a column of troops along the main road to Aleppo.
Colonel Abdel Jabbar al-Oqaidi, a spokesman for the rebel Free Syrian Army, told AFP news agency the troops were being redeployed to protect Aleppo because it was more strategically important.
The rebels have taken control of several posts along the Turkish border.
Turkey announced on Wednesday that it was shutting its border because of "security concerns".
It was unclear whether the move was related to the reported withdrawal of government troops.
Foreign journalists work under intense restrictions in Syria so reports by both sides are hard to verify.