Syrian activists say an attack by government helicopter gunships in the northern city of Aleppo has killed at least four people.

The Britain-based Syrian Observatory for Human Rights said Tuesday's attack struck an unused bridge, killing nine people and wounding many others.

Aleppo-based activist Baraa al-Halaby said four people were killed. He says nearly two dozen people have been killed in Aleppo, Syria's largest city, since the early hours in separate attacks.

Hundreds of people have been killed on both sides of the divided city since late April, as government forces have traded fire with rebels.

Rebels seized part of the city from forces loyal to President Bashar Assad in 2012. Assad's forces, backed by Russian airstrikes, have nearly encircled rebel-held parts of the city in recent months.