Syrian activists have raised the death toll from the late night airstrikes on the northwestern city of Idlib, saying at least 23 people were killed.

The Britain-based Syrian Observatory for Human Rights said Tuesday that Russian aircraft carried out the strikes, which pummeled the city.

Idlib is held by several militant groups, including al-Qaida's branch in Syria known as the Nusra Front. Another activist group, the Local Coordination Committees, gave a higher toll, saying the airstrikes killed 50.

Since the Russian military campaign began last September in an effort to shore up Syrian President Bashar Assad's forces in their battle against Islamic militants, Moscow has staunchly denied that its warplanes have hit any civilian areas in Syria.