US Secretary of State John Kerry arrived in Moscow on Tuesday to try and narrow differences with Russian leader Vladimir Putin over the role of Syrian President Bashar al-Assad in any political transition and which rebel groups should be part of peace talks.

Kerry 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 agreed in a phone call on the need for specific preconditions to be met before any new meeting, throwing the timing into doubt.

However, US State Department spokesman Mark Toner said there were no preconditions to having this meeting.

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

Speaking before Kerry's arrival in Moscow, a State Department official said Kerry would also raise concerns about Russia's continued bombing of Syrian opposition forces instead of Islamic State militants, an approach likely to anger Moscow.

Ahead of the talks, the Russian Foreign Ministry issued a statement complaining that Washington was not ready to fully cooperate in the struggle against Islamic State 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 Islamic State to negotiate with Damascus in Syrian peace talks.

While Kerry said there were still "kinks" that needed to be worked out, mainly to do with which groups should be included in peace talks, the Kremlin rejected the outcome of the Riyadh meeting, saying some of the groups were considered terrorists.

Assad himself appeared to cast doubt on the very idea of peace talks on Friday, saying he would not negotiate with armed groups that he said were backed by the US 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 January 1 ceasefire in Syria, as well as Monday's comments 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. News agencies on Monday quoted Valery Gerasimov, the chief of the Russian army's general staff, as saying his country was providing it with weapons, ammunition and material support, while a top Kremlin aide later said 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 November 24.

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 US maintains economic sanctions against Moscow for its annexation of Ukraine's Crimea region and support for separatists in eastern Ukraine. - Reuters