Opposition fighters in Syria are preparing for a fresh onslaught by government forces on rebel areas in the second city of Aleppo. Meanwhile, Turkey has closed all its border crossings with Syria.
The Free Syrian Army (FSA) said that a "large number" of regime forces had been moved from the province of Idlib to the second city.
Activists from the FSA were quoted by US news broadcaster CNN as saying that thousands of troops had withdrawn with their tanks and armored vehicles from the strategic Jabal al-Zawiya highlands near Turkey, sending them instead to Aleppo.
Meanwhile, the FSA's Military Council spokesman was reported by the AFP news agency as saying that the government needed to send in reinforcements because of the intensity of the fighting.
"There are clashes right now in Aleppo, so fierce that many of their troops are running away, while dozens of others are defecting on the spot," said Abdel Jabbar Oqaidi. "Their morale is very low."
An estimated 21 people were reported by the opposition Local Coordination Committees of Syria to have been killed in the city by helicopter and tank shells "falling on areas in the outskirts of Aleppo." The group said that the nationwide death toll for Tuesday stood at 133.
A commercial center with a population of 2.5 million, Aleppo recently became a new front in the country's 16-month uprising. Violence had previously been at a relatively low-scale, with the government maintaining a tight grip on security.
Early on Wednesday, troops loyal to President Bashar Assad fired artillery and rocket barrages at the northern Damascus suburb of Tel, according to residents and activists and rebels.
Meanwhile, Turkey said it was closing its frontier gates to cross-border traffic for an undisclosed period of time after a string of gates were seized by rebels.
State-run TRT television quoted Customs and Trade Minister Hayati Yazici as saying Turkey's 13 border posts along the 566-mile (911-kilometer) border would be closed to trucks heading for Turkish destinations as of Wednesday.
However, Yazici said that trucks traveling on to third countries would still be allowed through.
rc/kms (AFP, AP, dpa, Reuters)