A powerful explosion has hit Syria's northern city of Aleppo, close to the ruling party headquarters, the Syrian Observatory for Human Rights said.

“Initial details indicate that the Aleppo blast was targeting the local branch of the ruling Baath party and there is no information until now on the number of victims that fell in the explosion,” the Britain-based group said in an email.

Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.

It is not yet known if there are any casualties.