“Since this went into effect today at 00:01 in Damascus, we have seen an overall decrease in violence in these areas,” spokesman Mark Toner said.
“To ensure this continues in a sustainable way, we are coordinating closely with Russia to finalize enhanced monitoring efforts of this renewed cessation,” he said.
Last week, Washington and Moscow agreed to monitor a truce between Bashar al-Assad’s loyalist forces and opposition in rebels in Latakia and Eastern Ghouta.
But the divided city of Aleppo, a major commercial center in the north of the country, was excluded from their efforts and fierce fighting continued there.
Russian officials at first said that they would not try to rein in Assad’s forces, whom they said were targeting “terrorists” not party to the ceasefire.
But US Secretary of State John Kerry and UN peace envoy Staffan de Mistura have since petitioned Russia for a return to a nationwide truce agreed in February.
“We look to Russia as a co-chair of the International Syria Support Group to press for the Assad regime’s compliance with this effort,” Toner said.
“And the United States will do its part with the opposition,” he added.
“It is critical that Russia redouble its efforts to influence the regime to abide fully by the cessation.”
Once a nationwide “cessation of hostilities” is again in place, the United States and United Nations hope the warring parties will return to peace talks.
Kerry has warned that if Assad’s regime does not agree to begin a political transition away from his rule by August 1, it may face unspecified “repercussions.”
Reports in Washington suggest that this may mean the United States or its allies in the region are stepping up military supplies and training for the rebels.