BEIRUT (AP) -- Three suicide bombers detonated cars packed with explosives in the main square of the northern city of Aleppo on Wednesday, killing at least 33 people, leveling buildings and trapping survivors under the rubble, Syrian state TV said.
A fourth explosion a few hundred meters (yards) away also struck near the edge of the Old City, a UNESCO World Heritage site that has seen fierce street fighting between rebels and government forces.
There was no immediate claim of responsibility, but the government blamed its opponents and said the blasts were caused by suicide bombers. The bloodshed comes amid growing concerns that extremists such as al-Qaida are making inroads inside Syria.
The Syrian opposition denies any links to terrorists, but a Sunni extremist group called Jabhat al-Nusra, or Victory Front, has claimed responsibility for bombings in the past.
Long free of the violence that has engulfed much of the rest of the country, Aleppo in the past two months has become a key battleground between regime forces and rebels trying to oust President Bashar Assad. The opposition launched an initial offensive on the city, Syria's largest and a commercial hub, in July. Large swaths of the ancient city have been shattered.
Rebels last week announced a new concerted push to capture Aleppo, which would be a major strategic prize, giving the victor new momentum. It also would provide the opposition with a base and easy logistical supply lines with Turkey to the north that would allow them to carry out their fight against the regime in the rest of the country.
The bloodshed also is increasingly spreading outside Syria's borders.
On Wednesday, a shell fired from inside Syria landed on a home in neighboring Turkey, killing at least three people, including a 6-year-old boy, said Abdulhakim Ayhan, mayor of the Turkish town of Akcakale. Turkey's state-owned Anadolu Agency reported angry townspeople marched to the mayor's office to protest the deaths.
The uprising against Assad erupted in March 2011 and gradually morphed into a civil war. The conflict has killed more than 30,000 people, activists say.
Over the weekend, a fire sparked by fighting tore through Aleppo's centuries-old covered market in the Old City, burning more than 500 shops. At 12 kilometers (7.5 miles), it is the Middle East's longest souk and is part of Aleppo's old center that was added in 1986 to UNESCO's list of World Heritage sites.
Associated Press writers Zeina Karam in Beirut and Albert Aji in Damascus, Syria, contributed to this report.