The chance of unintentional confrontation between foreign powers involved in Syria has risen as its five-year-old conflict turns into a proxy war influenced by increasingly engaged outside powers, a European think-tank said on Wednesday.

Foreign countries are involved on both sides, with Russia and Iran deploying forces in support of President Bashar al-Assad and the US, Turkey and Saudi Arabia providing weapons, funds and political support to his rebel foes.

In a paper issued on Wednesday, the European Leadership Task Force recalled Turkey's downing of a Russian warplane that violated its air space in November and called on all states to avoid any actions that might be misinterpreted as a direct attack on other state's forces.

"According to the latest report of the U.N.-mandated Independent International Commission of Inquiry on Syria, the conflict has evolved into a 'multisided proxy war steered from abroad by an intricate network of alliances'," the paper said.

"In recent months, however, the danger of this proxy war turning into a direct interstate conflict has increased," said the security think-tank, whose experts include Russian former policymakers and European ex-ministers and defence planners.

"The shoot-down of the Russian Su-24 aircraft ... showed vividly that a direct confrontation is no longer inconceivable, even if the costs can potentially be catastrophic."

The United States and Russia have been struggling to restore an unravelling ceasefire and resurrect peace talks, which broke up in April when the opposition delegation walked out.

The report noted that the danger of accidental conflict had diminished after the ceasefire was reached in March.

But it said several countries seemed to assume they had space for assertive unilateral actions in Syria, as "the other side" would refrain from an actual confrontation.

"This assumption may be proven incorrect," it said. Countries involved in the war exercised limited control over the activities of their allies, while some might have an interest in pushing their sponsors towards interstate confrontation.

"Finally, there is a danger that an accident, military incident or an unauthorised action could spark a conflict, especially in the absence of effective communication channels."

The war has killed hundred of thousands, driven millions from their homes and created the world's worst refugee crisis.

The United States and Russia have taken the leading roles in diplomacy since Moscow joined the war last year with an air campaign that tipped the balance of power in favour of Assad. – Reuters