Earlier this year, at least. Kyiv Post, Ukrainian Special Forces and Syrian Rebels Decimate Russian Mercenaries in Syria. It may seem puzzling due to troop shortages along the front line in Ukraine, but the logic is similar to that framing UA’s Kursk incursion: The Syria operation could contribute to a significant distraction for Russian forces should the Syrian conflict re-ignite – which is has. More than that, action in Syria (and the Sahel) lends some credence to Kyiv’s insistence that it is fighting a global war for the West against Russia. In this story, cooperation with an Al Qaeda spinoff -Hayat Tahrir al-Sham (HTS) – a US-designated terrorist organization, is discounted. Also see:
Ukrainska Pravda, Ukraine plans attacks on Russian forces in Syria – The Washington Post
The Hill, Ukraine is taking the fight to Russia in Africa, Syria