Thousands of Syrian soldiers are being moved from the border with Turkey to join fierce fighting in the city of Aleppo, activists have said.
Rebels said their fighters had attacked columns of troops as they left 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.
Speaking amid news of the latest fighting, UN chief Ban Ki-Moon urged the world community to act immediately to stop the "slaughter" in Syria.
"I make a plea to the world: 'Do not delay... Act now to stop the slaughter'," the UN secretary general told parliament in Bosnia.
Until recently, Syria's 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 in Damascus.
And on Tuesday fighter jets strafed parts of Aleppo, with civilians and rebel fighters reported to be among the fatalities from fighting there.
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.
The BBC's Jim Muir in neighbouring Lebanon says the battle for Aleppo is almost as crucial for the regime's survival as the battle for the capital.
He says residents of Aleppo, a northern city regarded as the country's commercial hub, will know what to expect if they have seen what happened in Damascus.
Newly released video footage from activists shows streets in the capital's Qaboun quarter shattered by shellfire.
Other footage shows the corpses of large numbers of men discovered in several houses.
Fighting in both cities has been reported throughout Wednesday.
In other developments
- Human rights group Amnesty International says it is concerned about reports of "summary executions" by Syrian forces, calling them "serious violations of international law"
- Russia's foreign ministry says it has received "firm assurances" that regime's chemical weapons stockpile is "fully safeguarded"
- The new commander for the UN observer force, Lieutenant General Babacar Gaye, arrived in Damascus, saying he hoped his team would be able to help "alleviate the suffering"
- Activists say two more diplomats have defected to Qatar: Cyprus envoy Lamia al-Hariri and her husband Abdelatif al-Dabbagh, ambassador to the United Arab Emirates; one report says Mr Dabbagh was prevented from leaving Damascus
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.
Foreign journalists work under intense restrictions in Syria so reports by both sides are hard to verify.
Aleppo is just 60km (40 miles) from the Turkish border.
In recent weeks, the rebels have taken control of several posts along the 900km frontier, which has been plagued by attacks on lorries and fighting between rebels and government forces.
Turkey announced on Wednesday that it was shutting the remaining posts to lorries because of "security concerns".
Turkish officials said refugees would still be allowed through the border.
Analysts say the measure is an economic sanction, and that the embargo will hurt the regime, which relies on cross-border trade.
The BBC's Wyre Davies, at Killis, on Turkey's southern border with Syria, says there is a growing sense of anxiety and foreboding at the increasing violence inside Syria.
Turkey, once a close ally of Bashar al Assad, has made it clear it can no longer support a leader who is prepared to kill his own people in such large numbers and has called on the Syrian leader to stand down.