A day after a US Navy fighter jet shot down a Syrian warplane, a top Russian official called it an act of aggression that assists terrorists.

"This strike can be regarded as another act of defiance of international law by the United States," Russian Deputy Foreign Minister Sergei Ryabkov said Monday, according to Russia's state-run news agency Tass.

"What was it if not an act of aggression? It was also an act of assistance to those terrorists whom the United States is ostensibly fighting against," Ryabkov said.

The US military said that it shot down a warplane that had dropped bombs near Syrian Democratic Force (SDF) fighters. SDF forces are backed by the US-led coalition fighting ISIS.

It's the first time the US has shot down a Syrian aircraft since it began fighting ISIS in the country in 2014.

The strike came a little more than two hours after forces allied with the Syrian regime of Bashar al-Assad attacked the north-central Syria town of Ja'Din, which was controlled by the SDF.

A number of SDF forces were wounded in the attack, the statement from the Combined Joint Task Force said. The attack drove the SDF from Ja'Din, which is west of Raqqa, the coalition statement said.