UN chief Ban Ki-moon has urged world leaders to act to halt the "slaughter" in Syria, as thousands of troops reportedly move on the city of Aleppo.
"I make a plea to the world - do not delay... Act now to stop the slaughter...," Mr Ban said.
His comments came as activists said troops with tanks and armoured vehicles were redeploying to re-take areas of Syria's second city held by rebels.
Aleppo has seen fierce clashes, with fighter jets and helicopters deployed.
Meanwhile, government forces were reported to have largely repelled a rebel assault on Damascus, but there were further clashes there on Wednesday.
Activists said 23 people were confirmed killed in Damascus, most of them in a "massacre" which took place earlier in the Qaboun neighbourhood.
Video footage released by activists shows streets damaged by shell fire, and the corpses of large numbers of men discovered in several houses.
In other developments:
- Human rights group Amnesty International says it is concerned about reports of "summary executions" by Syrian forces and rebels, 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
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 - just 60km (40 miles) from Turkey - because it was more strategically important.
Foreign journalists work under intense restrictions in Syria so reports by both sides are hard to verify.
The BBC's Ian Pannell, who has been in Aleppo, says the battle for the city is one that neither side can afford to lose as the historic city is not only the largest in the country, but its economic heart.
On Tuesday fighter jets strafed parts of Aleppo, with civilians and rebel fighters reported to be among the fatalities from fighting there.
In recent weeks, the rebels have taken control of several posts along the 900km Turkish-Syria border, 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", but that refugees would still be allowed through.
Analysts say this is an economic sanction that will hurt the Syrian 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.
Turkey, once a close ally of President 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.