US Defense Secretary Ashton Carter said Monday that Russia is pursuing a "losing strategy" in Syria and that it must live up to its commitments in Ukraine.

US Defense Secretary Ashton Carter said Monday that Russia is pursuing a "losing strategy" in Syria and that it must live up to its commitments in Ukraine.

Speaking in Madrid at the start of a five-day trip to Europe, Carter told Spanish military officials that by carrying out bombing in Syria, Russia had only worsened the four-year conflict.

Russia last week started air strikes in Syria to eliminate the so-called 'Islamic State of Iraq and the Levant' (ISIL) takfiri group.

"Russia has escalated the civil war, putting further at risk the very political resolution and preservation of Syria's structure of future governance it says that it wants," Carter said.

"It remains my hope that Vladimir Putin will see that tethering Russia to a sinking ship is a losing strategy, and will decide to confront the threat presented by ISIL instead of continuing its unilateral airstrikes against Assad’s opposition."

Carter's trip is aimed at acknowledging allies in the 60-plus member US-led coalition that is carrying out daily drone and plane strikes against ISIL in Iraq and Syria.