The Pentagon chief says Russia is in pursuit of a "losing strategy" in Syria where Moscow’s anti-Daesh (ISIL) air campaign has only “escalated” the conflict on the ground.

US Defense Secretary Ashton Carter said Monday that "it remains my hope that [Russian President] 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 [Syrian President Bashar al-]Assad's opposition."

Carter was speaking in Madrid, Spain, at the start of a five-day trip to Europe, where he told Spanish military officials that by carrying out bombing in Syria, Russia had only worsened the 4.5-year-old conflict, according to the AFP.

Russia last week started airstrikes in Syria, with Moscow saying it was hitting ISIL targets, while the Pentagon claimed Russian jets were targeting Western-backed militants fighting the Syrian government.

"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 told participants at the Center for Strategic Studies and Defense in Madrid.

Carter will also attend a NATO summit in Brussels on Thursday, where several items including Russia's stance in eastern Ukraine will be discussed.

"We will continue to make it clear that if Russia wants to end its international isolation and be considered a responsible global power, it must stop its aggression in eastern Ukraine, end its occupation and attempted annexation of Crimea, and live up to its commitments under the Minsk agreements," Carter said.

"We will take all necessary steps to deter Russia's malign and destabilizing influence, coercion, and aggression, including its efforts to undermine strategic stability and challenge the military balance in Europe."

The West accuses Moscow of aiding pro-Russia forces in east Ukraine where a violent conflict between government forces and local fighters has taken the lives of thousands of people.