Russia strikes US-backed rebels in Syria: US official

The United States has called on Syrian President Bashar al-Assad to step down.

Washington: Russia carried out air strikes on Thursday in southern Syria against rebels battling Islamic State, including forces backed by the United States, a senior US defence official said, adding Washington would raise the matter with Moscow.

The official, who spoke on condition of anonymity, strongly criticised the Russian strikes near al-Tanf, noting that no Russia or Syrian ground forces were in the area at the time -- effectively ruling out an argument of self-defence.

"Russia`s latest actions raise serious concern about Russian intentions," the official said.

"We will seek an explanation from Russia on why it took this action and assurances this will not happen again.?"

Washington has consistently refused to join forces with Russia in Syria against Islamic State ever since Moscow launched its campaign of air strikes in September last year, accusing it of acting solely to prop up Syrian President Bashar al-Assad.

The United States has called on Assad to step down.

Communication between the US and Russian militaries on Syria has been limited to contacts aimed at avoiding an accidental clash as they carry out rival bombing campaigns and small numbers of US forces operate on the ground.