A US airstrike has hit pro-Syrian government forces in southern Syria, US officials say.

The war in Syria has been underway for more than six years, fought primarily between the government of President Bashar al-Assad and various forces opposing the government.

The airstrike targeted the Syrian regime forces after they breached a "de-confliction zone", according to CNN.

The US-led coalition commander first attempted to get the pro-Assad forces out of the zone by dispatching two US aircraft as "a show of force". When the forces did not respond, the commander called for an airstrike.

The strike was near a base in An Tanf, which is used by the coalition to train vetted Syrian opposition to help fight against Isis.

The coalition is generally not fighting the Syrian government, with the exception of a US cruise missile strike that was launched in retaliation for Assad's chemical weapons attack against civilians.