DAMASCUS (AFP) - The Syrian military for the first time on Wednesday deployed drones supplied by Russia for its fight against militants, a security source in Damascus said.

"For the first time, the army today used drones received from Moscow in operations against extremists in the north and east of the country," the source said, without elaborating on the type of drones or locating exactly where they were deployed.

The deployment came amid a Russian military build-up in Syria, over which the United States has expressed deep concern.

It also comes a day after a senior military official said Damascus had received new weaponry from Russia, including at least five fighter jets.

"Russian weapons are starting to have an effect in Syria," the official told AFP.

The official added that President Bashar al-Assad's forces had already been using these weapons against the Islamic State group in the cities of Deir Ezzor and Raqa, the Islamic State in Iraq and Syria (ISIS) de facto "capital" in Syria.

Assad's forces have stepped up air strikes in the past week, killing at least 38 ISIS militants in central Syria, especially Palmyra, according to the Syrian Observatory for Human Rights.