US Defense Secretary James Mattis Saturday said it is still uncertain who was behind a February 7th attack on US and US-backed forces in Syria.

The US official said he "understood" that Moscow had acknowledged Russian civilian contractors were involved. But he did not elaborate on how he came to that understanding. The US estimates that 100 pro-Syrian government forces were killed by US strikes to repel the Feb. 7th attack. It has declined to comment on the exact makeup of the attacking forces. Russian officials say five Russian citizens were killed in clashes with US-led coalition forces earlier this month. They also deny that they deploy private military contractors in Syria.