PARIS - Secretary of State John Kerry will go to Moscow on Tuesday for talks with Russian President Vladimir Putin about the war in Syria amid concerns about what Kerry termed "kinks" in an effort to get rebels and opposition groups to negotiate with the Damascus government.

"They will discuss ongoing efforts to achieve a political transition in Syria and related efforts to degrade and destroy ISIL," said State Department deputy spokesman Mark Toner, referring to the Islamic State.

Kerry also plans to raise with Putin and Russian Foreign Minister Sergei Lavrov the situation in Ukraine, where Russian-backed separatists and the government in Kiev are butting up against a self-imposed year-end deadline to end the fighting, Toner said. Kerry met with Putin once before this year, in the Black Sea resort of Sochi in May.

The pivot to the fighting in the Middle East and Eastern Europe comes as Kerry completes a week of talks in Paris over a climate-change accord.

On Sunday, he goes to Rome to cochair a summit on Libya, where the Islamic State has been making inroads, while U.N.-mediated talks aimed at forging a unity government have repeatedly fallen short. Italy, a former colonial power in Libya, fears that the Islamist group will send more migrants across the Mediterranean and use the North African country as a launching pad for terrorist attacks in Western capitals.

The war in Syria remains the most intractable problem and the focus of a large diplomatic push. On Monday, the French government is hosting foreign ministers from countries involved in Syria in one way or another. Later in the week, concerned nations hope to hold another meeting in New York to lay the groundwork for possible talks early next year between the government and its opponents.

More than 100 political and armed opposition groups met in Riyadh this week and agreed to a set of principles for negotiations that, at the core, would end with the departure of Syrian President Bashar al-Assad. The Syrian leader, however, still has solid support from Russia and Iran, and it is far from clear how an Assad exit could ultimately be finessed.

A large gap remains between those who say Assad should stay and all military efforts should be directed against the Islamic State and those who insist that Assad should leave power, preferably right away.

In a sign of infighting among Assad's opponents, the largest and most radical of the rebel groups, Ahrar al-Sham, briefly walked out of the Riyadh conference to show its discontent with other participants and the role that Islam would play in a future Syria.

Washington has expressed unhappiness at Moscow's military campaign in Syria, which has appeared to target Assad's opponents more than the Islamic State, although there are indications that that is changing.

In an interview with Spanish media published Friday by the official Syrian Arab News Agency, Assad rejected the idea of negotiations with the opposition groups that met in Riyadh, further complicating the international efforts to facilitate talks on a cease-fire and political transition.