The United States has expressed deep concern about Russia’s moves to support Syrian strongman Bashar al-Assad’s regime, but has opened talks with Moscow on how their forces can avoid getting in each other’s way.

A US official, speaking on condition of anonymity, confirmed reports that four Russian jets had been seen on the ground at Latakia air base in western Syria, the heart of a Russian force build-up in the country.

Both Moscow and Washington say they want to fight the Islamic State jihadist group in Syria but differ over how to treat Assad’s regime, which Russia sees as a bulwark against the extremists.

The United States and its Arab allies blame Assad for triggering a civil war within his own country through persecuting Syrian civilians and thus creating the chaos that has allowed the IS revolt to flourish.