At least 40 people were killed and 90 wounded in a series of explosions in the center of the northern Syrian city of Aleppo on Wednesday, the Syrian Observatory for Human Rights said, citing medical sources.

State television said four blasts ripped through Aleppo's main Saadallah al-Jabiri Square and a fifth struck a few hundred meters away, on the fringes of the Old City where rebels and forces loyal to President Bashar Assad have been fighting.

There were no immediate details on the nature of the blasts, which activists said were likely caused by car bombs.

The northern city of Aleppo, Syria's commercial hub and largest city, has seen intensified fighting between regime forces and rebels trying to oust President Bashar Assad, especially after the fighters launched a new offensive last week.

Syrian TV reported the triple bombings at the Saadallah al-Jabri square, describing them as the work of "terrorists." Authorities refer to rebels fighting to topple Assad as terrorists and armed gangs.

The TV did not provide further details.

Aleppo-based activist Mohammad Saeed said the explosions went off minutes apart at one of the city's main squares. He said the blasts appear to have been caused by car bombs and were followed by clashes and heavy gunfire.

The area is controlled by government forces and there is a military club nearby which activists said may have been the target.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, reported dozens of casualties from the blasts, most of them members of the regime forces.

In a statement, it said the explosions went off following a clash between guards at the military club and gunmen, suggesting the attacks may have been suicide bombings.

Suicide and car bombings targeting security agencies and soldiers have become common in Syria, particularly in the capital, Damascus, during the course of the 18-month-uprising against Assad.

But such bombings have been rare in Aleppo, which was spared the mayhem that struck other Syrian cities during the first year of the revolt. Then, in February, two suicide car bombers hit security compounds in Aleppo's industrial center, killing 28 people.

The uprising against Assad that erupted in March 2011 ago has gradually morphed into a bloody civil war. The conflict has killed more than 30,000 people, activists say, and has devastated entire neighborhoods in Syria's main cities, including Aleppo.