Heavily armed government troops have been steadily shelling rebel-controlled parts of the city, particularly Salaheddine and other districts on the southwestern edge of Aleppo as the two sides fight for control over the strategic city.
Aleppo-based activists said clashes were going on Tuesday near the historic city center in Bab Jnein and Sabee Bahrat districts, suggesting the rebels were making some inroads. They said intense bombardment of the town of Tal Rafaat near the Turkish border sent scores of people spilling into Turkey for safety.
A Turkish government official said 1,328 Syrian refugees had crossed the border by midmorning - nearly double the number of refugees who reached Turkey on Monday. The official spoke on condition of anonymity in line with government rules.
The new arrivals would increase the number of Syrians who have found refuge in Turkey, which has served as a staging ground for rebels fighting Assad's regime, to close to 48,000.
"We are expecting a massacre in Aleppo. The regime is bringing reinforcements to the city because they consider that if Aleppo falls, the regime will fall," said a Syrian refugee in Turkey who identified himself as Abu Ahmad.
"The city is being bombed from the air and ground," he said, adding he was in daily contact with residents still in the city.
The rebels appeared also to be bringing in reinforcements.
A video posted online by activists Tuesday showed a large group of Free Syrian Army rebels in military fatigues and carrying shoulder carried rocket propelled grenades and automatic rifles, announcing they were joining the "Unification Brigade," the main group of rebel fighters in Aleppo, to assist in the "liberation" of the city.
"They have Satan on their side, we have God on ours," one rebel shouts. "We are coming, Aleppo," shouts another. The authenticity of the video could not be independently verified.
Despite a ferocious crackdown, rebels in Syria have grown more confident and are using increasingly bolder tactics both in Aleppo and the Syrian capital, Damascus.
In a brazen, daylight attack, rebels on Saturday abducted a group of 48 Iranians near Damascus, branding them as spies assisting in Assad's crackdown against the rebels.
Iran said those captured when their bus was commandeered were pilgrims visiting a Shiite shrine on the outskirts of Damascus. On Tuesday, Iran's Foreign Ministry said it holds the US. responsible for the fate of the abducted Iranians.
Iran's state IRNA news agency said the ministry summoned the Swiss envoy in Tehran late Monday to stress that Iran expects Washington to use its influence to secure the Iranians' release. The Swiss look after U.S. interests in Iran since Tehran and Washington have no diplomatic relations.
The abductions threaten to suck Iran deeper into Syria's civil war and the wider political brinksmanship around the region. Iran says it has no fighting forces aiding Assad, but it has sharply amplified its criticism of countries supporting the rebels such as neighboring Turkey and Gulf states led by Qatar and Saudi Arabia.
The Iranian Embassy in Turkey said Iran's Foreign Minister Ali Akbar Salehi is traveling to Turkey and will meet Turkish Foreign Minister Ahmet Davutoglu in Ankara on Tuesday to discuss Syria and the situation of the abducted Iranians.
In Damascus, a senior Iranian envoy, Saeed Jalili, met with Assad on Tuesday, according to state-run news agency SANA. State TV showed a picture of Assad sitting across from Jalili at the presidential palace - the president's second appearance on TV since a July 18 bombing in Damascus that killed four of his top security officials, including his brother in law, during a rebel assault on the capital.
Assad has not spoken publicly since and his low profile has raised questions on whether he fears for his personal safety as the civil war escalates dramatically.
Syrian rebels, meanwhile, claimed three of the Iranian captives were killed on Monday during shelling by government forces in Damascus and its suburbs, and threatened to kill the remaining Iranians unless the army stopped its bombardment.
"The Syrian regime is responsible for anything that happens to the Iranians," a spokesperson from the Baraa Brigades that claimed responsibility for the group's abduction told The Associated Press on Skype.
However the spokesperson's claim that three were killed could not be independently verified. An official at the Iranian Embassy in Damascus said he had no information on the subject.
While skirmishes were reported in the Syrian capital overnight and its suburbs Tuesday, the main battle has now moved to Aleppo, some 350 kilometers (215 miles) north of Damascus where rebels seized several neighborhoods two weeks ago and have proved difficult to dislodge.
Aleppo is Syria's commercial hub and it's close to the Turkish border where the rebels have their rear bases. If the opposition were to gain control, it would be a major blow to the regime and a possible opposition base of operations.
"The rebels are making their way closer to the city center despite the bombing using everything from planes and helicopters to artillery shells and machine guns," Hazem, the activist, said.
He said clashes were also reported around the medieval citadel, a symbol of the city that dominates its ancient center.
The Britain-based Syrian Observatory for Human Rights also says fighting Tuesday has extended to new parts of the city.
Ahmad Saleh, a Syrian from the town of Tal Rifaat near the Turkish border, said the town was shelled Monday from the nearby air base of Minnegh, killing two people. In addition to its own inhabitants, many of Aleppo's residents had fled to Tal Rifaat were they were staying with relatives or friends.
Saleh, who fled to Turkey after midnight Tuesday, said pharmacies and grocery shops are empty and it is difficult to buy such products.
"The situation is miserable and it is not possible to find goods," said the man. "We had to choose between dying in Syria or coming to Turkey."