Deadly airstrikes by the international coalition on pro-regime forces in eastern Syria were purely an act of self-defence, Pentagon chief Jim Mattis said Thursday (February 8th), AFP reported.

"It was self-defence," Mattis said. "We are not getting engaged in the Syrian civil war."

More than 100 pro-regime fighters were killed when the coalition struck back against an attack on the Syrian Democratic Forces (SDF) in Deir Ezzor province.

Calling the incident "perplexing", Mattis said "for some reason, pro-regime forces -- and again, I cannot give you any explanation of why they would do this -- moved against SDF positions" where US special forces also were present.

These forces began shelling with artillery, he said. "They were moving with tanks, obviously in the same direction as they were firing. At the end of our effort to defend ourselves, their artillery was knocked out, two of their tanks were knocked out, they had casualties."

The coalition quickly contacted its Russian counterparts to attempt to ensure the situation did not escalate, Mattis said, adding: "The Russians, at that time, were telling us they did not have forces there."

The fighting took place less than 10 kilometres east of the "deconfliction zone" set up by the US and Russia along the Euphrates River.

Moscow and Washington co-ordinate their air and ground operations against the "Islamic State of Iraq and Syria" (ISIS) in this zone.

Mattis said the coalition had picked up on the movements of pro-regime forces in the area a few days before, but they were not immediately hostile.