Agreement was reached late on Tuesday: a shaky ceasefire is to be extended to cover the embattled northern city of Aleppo, in Syria.
US Secretary of State, John Kerry says the truce is vital for a peaceful future in Syria.
"We just announced a short time ago, as part of urgent efforts to try to de-escalate the violence in Syria and reaffirm the cessation of hostilities which is critical, that the United States and Russia have now concluded arrangements with our teams, late yesterday evening, to reaffirm the cessation of hostilities in Aleppo province, including Aleppo city and its surrounding areas."
Mr Kerry has urged Russia to continue to press Syrian President Bashar al-Assad to abide by the agreement, while Washington will deal similarly with Syrian opposition forces.
But Russian Foreign Minister Sergey Lavrov says the relationship between Russia and Syria isn't that close.
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"I often hear our American partners call on us to work more actively with Bashar al-Assad, we do by the way, but not as an ally. We support him in the fight against terrorism and in the preservation of the Syrian state. But he is not an ally of Russia in the same way Turkey is an ally of the US."
Aleppo has seen a renewed surge in fighting in recent days, endangering the first major ceasefire during Syria's five-year civil war brought in in February.
Hospitals and civilian areas have complained they have come under repeated attack.
Reports have differed over the truce's start, with the U-S stating it would come into effect from 12.01 am on Wednesday Damascus time.
The Syrian military says Thursday will be the first day of the so-called "regime of calm", and it will last just 48 hours.
The armistice comes after reaffirmations that truces in the Damascus suburbs and Latakia province would continue.
As previously, it will not apply to the self-proclaimed Islamic State or the al-Qaeda-affiliated Al Nusra Front.
The United Nations Humanitarian Chief, Stephen O'Brien, has told the U-N Security Council, the cessation in fighting is an opportunity for renewed hope of a permanent end to the violence.
"Life for people in Aleppo is horrendous and has lost all sense. People are living under daily threat and terror. The carnage in Aleppo and in the wider Syria must stop. And for the sake of the people of Aleppo, Syria and beyond, that we cannot squander the opportunity of the negotiations in Geneva."
Earlier, French and German Foreign Ministers met with Syrian negotiators and UN envoy Staffan de Mistura, in an effort to break a deadlock in peace talks.
A meeting of the 17-nation International Syria Support Group is expected to take place within weeks.
