As many as 275,000 people have been under siege in eastern part of city since September
Syrian rebel fighters have launched an offensive against government forces in Aleppo to try and break a siege which has lasted for months.
There are conflicting accounts of advances after hundreds of missiles as well as suicide car bombings targeted the western edge of the city.
A Syrian military airport was also under attack.
The UK-based Syrian Observatory for Human Rights (SOHR), which is monitoring the fighting, said at least 15 civilians had been killed and 100 wounded by rebel shelling of the government-held west of the city.
Syrian state media said five people had died.
In another development, the Russian Defence Ministry said its general staff have asked President Vladimir Putin for permission to resume airstrikes against militants in eastern Aleppo after a 10-day lull.
The Interfax news agency said the ministry wants to resume bombings because of an increase in militant activity and continuing civilian deaths.
It is unclear if President Putin will agree to the request.
Aleppo has been divided since 2012, and the rebel-held east has been battered by months of Syrian and Russian airstrikes which are reported to have killed hundreds of people.
Rebels claimed to have seized several positions from government forces on Friday and SOHR said they had gained control over a checkpoint at a factory in the southwest of the city, as well as some other points nearby.
But a Syrian military source said the army and its allies had foiled what he called "an extensive attack" on south and west Aleppo by rebel groups and Islamic State.
State television reported that the army had destroyed four car bombs.
As many as 275,000 people have been under siege in eastern Aleppo since September.
Airstrikes were last halted last month so that humanitarian aid could be delivered.
The offensive has so far been carried out by rebels outside Aleppo, but it is thought groups inside the city will soon join the fighting.
President Assad's forces, backed by Russia, Iran and Shia militias, are facing opposition from the Free Syrian Army (FSA) and groups supported by Turkey, the United States and the Gulf monarchies.
Among the rebels are members of Jabhat Fateh al-Sham, formerly known as the Nusra Front, which the US has distanced itself from because of its connections with al-Qaeda.
Abu Anas al-Shami, a spokesman for the group said it had carried out two "martyrdom operations".
Fighters had then gone in and had been able to "liberate a number of important areas", he said.
A third suicide bombing is reported to have been carried out by another Islamist group.
A senior official in the Levant Front, an FSA group, said: "There is a general call-up for anyone who can bear arms.
"The preparatory shelling started this morning."
The rebels tried to break the siege of Aleppo in August, and temporarily opened a corridor between the east and west.
Aleppo was once a bigger city than the capital Damascus, and was Syria's economic hub.
Since airstrikes intensified up to 500 people have been killed and 2,000 injured, prompting UN human rights chief Zeid Raad Al Hussein to describe the city as a "slaughterhouse".