US Secretary of State John Kerry has flown flew into Moscow to meet Russia's President Vladimir Putin and urge him to keep the fledgling Syrian peace process on track.
Washington is relying on the Kremlin to drag Russian ally Bashar al-Assad to the negotiating table for talks with his rebel opponents to end Syria's vicious civil war.
In turn, US ally Saudi Arabia is putting together a rebel coalition that would sit down with Assad's negotiating team, sign a ceasefire and start a political dialogue.
Looming over the effort to end the bloody four-and-a-half year civil war is the threat posed by the Islamic State group to spread the carnage beyond Syria's borders.
The hope is that if the regime and the rebels can agree on a ceasefire then they, Russia and a US-led coalition of Western and Arab allies can focus their fire on IS.
Washington and Moscow are the key powers in the process, leading talks through the 17-nation International Syrian Support Group in co-operation with the United Nations.
Kerry and the UN envoy for Syria, Staffan de Mistura, want to hold the next round of ISSG talks on Friday in New York, but Moscow has so far refused to confirm the date.
At issue is the Saudi-mediated rebel delegation and a separate effort by Jordan to draw up a list of Syrian groups deemed "terrorists" and barred from the talks.
Moscow said on Monday that Friday's talks should not take place until both the rebel representation and the terror blacklist are agreed upon by all parties.
But Washington insists it is vitally important to maintain the momentum of the process rather than squabbling over the detail, fearing that Putin is trying to buy Assad time.
Meanwhile, back in Washington, US President Barack Obama voiced fresh determination to destroy the Islamic State, vowing to kill the group's leaders and win back territory in the Middle East.
On the ground in Syria, a military source said government troops recaptured a military airport and town east of Damascus, more than three years after they were overrun by rebel groups.
The war began with anti-government demonstrations in March 2011 but evolved into a multi-front civil war that has killed more than 250,000 people and forced millions from their homes.