Syrian activists say two powerful explosions have rocked the center of the northern city of Aleppo, causing casualties and heavy material damage.

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

The area is controlled by Syrian government forces. The target of the explosions was not immediately clear.

The Britain-based Syrian Observatory for Human Rights reported three powerful explosions at the main square in Aleppo but had no word on the number of casualties.

Aleppo is Syria's commercial capital and there has been heavy fighting there for days between government troops and rebels trying to oust President Bashar Assad.