The blast happened as the regime and opposition resumed population exchanges four days after a deadly attack on a convoy of evacuees forced both sides to suspend the deal.
A bomb killed six people and wounded more than 30 in Syria's second city Aleppo on Wednesday, the Assad regime's official television channel reported.
The blast hit the southwestern neighbourhood of Salaheddin, which was on the front line between opposition and regime-held sectors before Bashar al Assad's forces retook full control of the city in December.
The attack took place as evacuations of civilians from conflict zones resumed after a four-day pause.
About 45 buses carrying 3,000 people left the opposition-besieged al-Foua and Kefraya towns of the northwestern Idlib province for regime-controlled Aleppo.
Meanwhile, a convoy of 11 buses left regime-besieged al-Zabadani, near the capital Damascus, for opposition-controlled territory, a military media unit linked to the pro-regime Hezbollah group said.
"The area of Zabadani is empty of militants after the last batch of them left this morning," the pro-regime Sham FM radio reported, citing a senior regime official in Zabadani.
Three buses on Wednesday also carried people who were injured in a convoy attack as they were being transferred from al-Foua and Kefraya on Saturday, as well as the remains of those who had died in the attack, the Hezbollah military media unit said.
Saturday's convoy attack killed 126 people, including more than 60 children, the UK-based Syrian Observatory for Human Rights said.
The convoy was carrying at least 5,000 people, including civilians and several hundred pro-regime fighters, who were granted safe passage out of the two towns.
The attack forced a population exchange deal, brokered by regime ally Iran and opposition backer Qatar, to be halted.
Under the deal, more than 30,000 people will be evacuated from four Syrian towns in two stages.