U.S. Secretary of State John Kerry addresses journalists during a news conference at the ministry in Athens, Greece, Dec. 4, 2015.

U.S. Secretary of State John Kerry will use talks in Moscow on Tuesday to try to narrow differences with Russian leader Vladimir Putin over the role of Syrian President Bashar al-Assad in any political transition, a senior State Department official said on Monday.

He will seek to prepare the ground for a third round of talks of world powers on Syria amid doubts over whether a meeting pencilled in for Friday in New York will go ahead.

Russia’s foreign ministry said late on Monday that Kerry and Russian Foreign Minister Sergei Lavrov had agreed in a phone call on the need for specific preconditions to be met before any new meeting, throwing its timing into doubt.

Russia is one of Assad’s staunchest allies and launched a campaign of air strikes to support his forces against insurgents on Sept. 30. It says only the Syrian people and not external powers should decide his political fate.

Speaking in Paris before Kerry visits Moscow on Tuesday, the State Department official said Kerry would raise concerns about Russia’s continued bombing of Syrian opposition forces instead of Islamic State of Iraq and Syrian (ISIS) militants, an approach likely to anger Moscow.

Ahead of the talks, the Russian foreign ministry issued a statement criticizing U.S. policy on Syria, complaining that Washington was not ready to fully cooperate in the struggle against ISIS militants and needed to rethink its policy of “dividing terrorists into good and bad ones”.

Kerry’s meeting with Putin follows a meeting last week in Riyadh which agreed to unite a number of opposition groups excluding ISIS to negotiate with Damascus in Syrian peace talks.

While Kerry said there were still “kinks” that needed to be worked out on the plan to unite the opposition groups, the Kremlin rejected the outcome of the Riyadh meeting, saying it had no right to speak on behalf of the entire Syrian opposition.

Assad himself appeared to cast doubt on the very idea of peace talks last Friday, saying he would not negotiate with armed groups that he said were backed by the United States and Saudi Arabia.

The opposition groups said Assad should leave power at the start of a transitional period.

No deals on Ukraine

“We don’t have a full meeting of the minds yet (on Assad), we will talk about some of the details of a transition…in the hopes of narrowing the differences between us,” a senior State Department official told reporters.

While in Paris on Monday, Kerry met with counterparts from Saudi Arabia, Qatar and Jordan as he prepared for the Moscow talks.

His talks with Putin, and a lengthier meeting in the morning with Lavrov, will delve deeper into details of a planned Jan. 1 ceasefire in Syria, as well as recent statements by Russia on supporting the Free Syrian Army (FSA), which has been fighting Russian- and Iranian-backed government forces.

“We’ll be interested to hear what the Russians have in mind there, given the Free Syrian Army’s concern about how Assad has been treating his own people,” the official added.

Russia has given mixed messages on the FSA with news agencies on Monday quoting Valery Gerasimov, the chief of the Russian army’s general staff, as saying his country was providing it with weapons, ammunition and material support, and a top Kremlin aide later saying there was no such arrangement.

FSA rebels fighting Assad’s forces in western Syria denied receiving any support from the Russian air force, saying on the contrary it continued to bomb them.

White House spokesman Josh Earnest said Kerry will encourage continued efforts by Russia to de-escalate tensions with Turkey after it shot down a Russian military plane near the Syria border on Nov. 24.

“We haven’t seen any tangible escalation at this point,” Earnest said at a news briefing. “So that’s, you know, a glimmer of optimism in that situation.”

Kerry, during his meetings, will also underscore the importance of implementing the Minsk peace accords on Ukraine and plans for free and fair elections in eastern Ukraine.

Despite cooperation with Russia on a political transition in Syria, the United States maintains economic sanctions against Moscow for its annexation of Ukraine’s Crimea region and support for separatists in eastern Ukraine.

“We have made absolutely clear all the way through this at every level that we are not playing ‘lets make a deal here’ trading Ukraine for Syria, that these are distinct issues with distinct paths forward,” the official added.

Related articles across the web

• The Volunteer Firefighters Of Daraa (Excerpt From 'The Battle For Syria's South')
• Israeli, Russian Deputy Military Chiefs to Coordinate on Syria
• Putin moves forward against Islamic State in Syria
• Netanyahu, Putin Aim to Prevent Accidental Israeli-Russian Clashes in Syria
• Peskov: President al-Assad can't be excluded, Merkel's statements match
• Not Losing Face in Syria
• Israel, Russia to work in tandem
• The Syrian Powder Keg: Markets Getting Explosive
• Bashar al-Assad Had a Great InstaSummer While His People Drowned in the Mediterranean
• Syria's Rebels Respond To The Russian Military Build-up And Moscow's Support Of Assad