The two countries ended talks Tuesday and are closely monitoring whether the ceasefire sticks. Violence has decreased but continues, according to State Department deputy spokesman Mark Toner.
"Attacks directed against Syria's civilian population can never be justified, and these must stop immediately," Toner said in a statement Wednesday.
"Our objective remains, and has always been, a single nationwide cessation of hostilities covering all of Syria -- not a series of local truces."
The ceasefire will last for 48 hours, Syria's army's general command said. The truce started at 1 a.m. local time Thursday.
Russia's air base in Syria
Russian fighter jets continue to roar off the tarmac at Hmeymim air base in Latakia, the staging ground for Russia's air war in Syria.
Nearly two months after Russian President Vladimir Putin announced he was pulling out "the main part" of his country's forces
from Syrian territory, strike aircraft, supported by fighter jets, keep taking off and landing with regularity.
The air base, situated on northwest Syria's Mediterranean coast, remains a potent military concern. While some of the fighter jets have been removed, Russian military officials have upgraded their helicopter fleet at the base, adding Mi-28 helicopters that are on constant patrol.
The remaining fighter jets and strike aircraft show how the Russians are still capable of playing a decisive role in the Syrian conflict.
Maj. Gen. Igor Konashenkov, a spokesman for the Russian Defense Ministry, said that Russian jets have flown 87 sorties striking ISIS in the last four days alone.
The strikes have targeted the terror group's assets in Raqqa and Deir Ezzor provinces, he said.
He says that Russia has been working with the U.S. to ensure the ceasefire in Aleppo works. He adds that so far this cooperation has been positive -- the coordination of the two forces' air operations is going well and over the past month the forces have shared "a lot of information."
Russia has said it wants to see the "regime of silence" that has taken hold in Latakia and the capital, Damascus, extended to Aleppo.
But monitoring group the Syrian Observatory for Human Rights said that ongoing clashes in western Aleppo between the Syrian army and rebel factions have been the fiercest in the past year.
Urgent Security Council meeting
The talks between Russia and the United States are part of an international effort to enforce a cessation of violence across Syria. The U.N. Security Council convened an urgent meeting Wednesday on the violence in Aleppo in response to a request from Britain and France.
Matthew Rycroft, the UK ambassador to the United Nations, had strong words for the situation.
"This council has an obligation to the people of Aleppo," he said. "We have an obligation to show that we are working for their protection; that we are trying to find that elusive political settlement that will end this war."
He added that the Syrian people had lost confidence in the international body, and to restore their faith the United Nations needs to stop the carnage.
Jeffrey Feltman, the under-secretary of political affairs, told the council that the cessation of hostilities needed to be put back on track. He noted the agreement between Russia and the United States.
"We urge parties to abide by this immediately and comprehensively," he said.
He blamed both sides for the violence -- the Syrian government for bombing and the opposition for shelling.
France will organize a ministerial meeting on the situation in Syria on Monday, U.N. Ambassador Francois Delattre said in New York.
He said France also is asking Syria's allies to "use all their influence on Damascus in order to end the fighting and in order for the peace process to resume and to resume quickly."
U.S. officials have called on Moscow to pressure the Syrian President to end his onslaught on Aleppo, which has resulted in widespread deaths of civilians in what was once the country's bustling cultural and economic hub.
Fighting in Aleppo
Dozens have been killed on both sides in the latest surge of violence as rebel groups have shelled government-controlled areas of Aleppo and the Syrian military has responded, the observatory said.
Rocket attacks targeting government-held areas in Aleppo killed at least 17 people Monday
, state media reported.
Fatalities included people at al-Dhabit hospital, state media and the Syrian Observatory for Human Rights reported, blaming Islamist groups for the shelling.
The attacks came days after an airstrike hit a hospital in a rebel-held area of Aleppo, reportedly killing the city's last pediatrician. The death toll from last week's attack has risen to 55, according to Doctors Without Borders.
"The bottom line is there is no justification for this horrific violence that targets civilians or medical facilities or first responders no matter who it is, whether it's a member of the opposition retaliating or the regime in its brutality against the civilians which has continued for five years," U.S. Secretary of State John Kerry said.
Elsewhere, jets conducted at least 22 airstrikes Wednesday in Eastern Ghouta near Damascus, ending a truce in the area the military had announced Friday, according to the observatory.