Moscow and Washington made a joint call Monday for Syria's regime and rebels to agree to ceasefires in parts of their battle-scarred country ahead of peace talks this month.
But the two countries continued to disagree on Iran's participation in the talks, with US Secretary of State John Kerry saying Tehran needs to accept plans for a transitional government if it wants to take part.
Meeting in Paris with UN-Arab League envoy Lakhdar Brahimi, Kerry and Russian Foreign Minister Sergei Lavrov called for the ceasefires to start ahead of the so-called Geneva II talks due to begin in Montreux on January 22.
It was a rare pulling together of two heavyweights in the Syrian crisis, which has seen the US backing the opposition as Moscow sticks by longtime ally President Bashar al-Assad.
"We talked today about the possibility of trying to encourage a ceasefire, maybe a localized ceasefire beginning with Aleppo (in the north of Syria)," Kerry said.
"What can be done before the beginning of the conference should be done," Lavrov said. "We are going to try to send signals to all the Syrian sides on the need for the establishment of a localized ceasefire."
Washington and Moscow are hoping to build momentum ahead of the talks, which are seeking to revive efforts to hammer out a deal on a transitional government after a nearly three-year conflict that has killed 130,000 people.
Damascus's ambassador to Moscow said Monday that Syrian Foreign Minister Walid Muallem will also visit Russia ahead of the talks, with a diplomatic source saying the visit could take place on Thursday.
Kerry and Lavrov said they also discussed possible prisoner exchanges and the opening of humanitarian corridors to bring aid to the millions affected by the conflict.
Lavrov said the Assad regime had indicated it was ready to allow access for humanitarian aid, citing the embattled Damascus suburb of East Ghouta, where thousands have been trapped by fighting.
"We expect similar steps from the opposition," Lavrov said.
'Success is defined by a good beginning'
The two said they hoped these issues could be sorted before the talks, but that they would not be preconditions for sitting down.
"Success is defined by a good beginning," Kerry said.
But divisions remained on the role of Syria's main regional backer Tehran, with Lavrov saying there was an "absolutely apparent need" for the participation of both Iran and Saudi Arabia.
Brahimi said he also supported Iran's involvement. But Washington has blocked efforts to extend an invitation to Tehran until it agrees to the idea of a transitional government set out in the first Geneva negotiations.
Kerry said Iran would be "welcome" if it agreed to the Geneva I deal, adding: "I invited Iran today to join the community of nations... and be a constructive partner for peace."
Iranian Foreign Minister Javad Zarif heads to Moscow Thursday for talks with Russian President Vladimir Putin on Iran's nuclear program and on regional issues including Syria.
Zarif, who said on Sunday that Tehran would only take part in the Syria talks "without any preconditions," is also due to travel to Damascus in the next few days.
The Assad regime has agreed to send a delegation to the talks but Syria's deeply divided opposition has struggled to agree on attending.
After a meeting in Paris Sunday of the "Friends of Syria" group of mainly Western and Gulf countries opposed to Assad, Kerry said he was "confident" the opposition would attend.
Kerry was due to meet again Monday with opposition National Coalition leader Ahmad Jarba, after similar talks on Sunday.
Syria warned against preconditions for the peace talks, after Jarba said Sunday that Assad's departure is "inevitable."
"Any person who seeks preconditions or mistakes their dreams for reality is leading to the failure of the Geneva conference before it even starts," state media quoted a Syrian foreign ministry source saying.
The meetings in Paris come amid days of fighting pitting moderate and Islamist opposition forces against the Al Qaeda inspired Islamic State of Iraq and the Levant (ISIL) that has killed nearly 700 people.
After Paris, Kerry will head to the Vatican Tuesday to meet top officials for talks on Middle East peace efforts, before heading to Kuwait City for a conference where the United Nations is seeking $6.5 billion (4.7 billion euros) to aid Syrians affected by the war.