Bomb attacks have grown more common in Syria's two largest cities as the uprising against President Bashar Assad grows increasingly militarized. Many in the opposition have taken up arms since protesters first took to the street in March 2011 and now regularly clash with government forces around the country.
But Aleppo and Damascus have remained largely in Assad's grip, shaken only by bomb blasts that often appear to target buildings associated with the military and security services.
The U.N. says more than 9,000 people have been killed since the uprising's start.
Saturday's blast in Aleppo, Syria's largest city, hit a car wash and killed six people, Aleppo activist Mohammed Saeed said via Skype. He said the business in the city's southern Sukari neighbourhood is owned by a man who serves in pro-government militias known as the shabiha.
The Britain-based Syrian Observatory for Human Rights, which relies on activists inside Syria, said five people were killed in the attack.
The blast follows increasing unrest in the city with university students taking to the streets and being violently dispersed by security forces.
A 16-year-old was shot dead during a protest Friday, one day after four students were killed during arrest raids in university dorms.
Also Saturday, an explosive planted under an army vehicle in Damascus blew up, damaging nine cars.
The blast shook a downtown neighbourhood near a military food co-operative, and left a crater in the street, according to a reporter from The Associated Press who visited the scene.
No one immediately claimed responsibility for the explosions.
Earlier this week, attacks on a government security compound and the country's central bank killed nine and injured 100.
On the outskirts of the capital, tanks and troops combed through fields near the Barzeh neighbourhood in the northeast as well as an area near Hamouriya in the east, activist Omar Hamzeh said via Skype.
"They are moving through the fields and firing anti-aircraft guns," he said, adding that the regime was probably looking to arrest activists. "The rural areas around there have been very active in protests against the regime."
The Observatory also reported army raids in Barzeh.
Activist claims could not be independently verified. The Syrian government prevents most media from working freely in the country.
World powers remain divided on how to stop Syria's crisis, though all have fallen in behind a plan put forward by former U.N. Secretary General Kofi Annan that seeks a cease-fire in order for all sides to engage in political dialogue.
But the truce that was to begin on April 12 has never really taken hold, with regime forces continuing to shell opposition areas and shoot at protesters. Security forces killed one teenager following a protests in the northern city of Aleppo on Friday.
Armed rebels have kept up attacks on military checkpoints and convoys.
A spokesman for Annan, however, said Friday the international envoy believes his peace plan for Syrian remains "on track" — a day after the Obama administration offered a far bleaker view, saying the plan might be doomed.
A U.N. team of up to 300 members is to monitor compliance with the truce. U.N. peacekeeping chief Herve Ladsous said about 40 U.N. observers are on the ground in Syria and that the force will grow to 65 by Sunday.
Associated Press writer Albert Aji contributed reporting from Damascus, Syria.