Syria's fragile cease-fire started to unravel on Sunday with the first aerial attacks on rebel-held neighbourhoods of Aleppo and a southern village that killed at least eight people.
The violations that came as tensions between the American and Russian brokers of the deal worsened following a deadly US strike on Syrian government forces.
The air raid by the US-led coalition killed dozens of Syrian soldiers and led to a harsh verbal attack on Washington by Damascus and Moscow.
The US military says it may have unintentionally struck Syrian troops while carrying out a raid against the Islamic State group in eastern Syria on Saturday.
The seven-day cease-fire is supposed to end at midnight on Sunday, according to a Syrian army statement issued last week.
The US and Russia have said that if it holds for seven days, it should be followed by the establishment of a Joint Implementation Centre for both countries to co-ordinate the identification of targets against the Islamic State and al Qaida-linked militants.
Despite largely holding, the ceasefire has been repeatedly violated by both sides, and aid convoys have not reached besieged rebel-held neighbourhoods of Aleppo, Syria's largest city and one-time commercial centre that has been the centre of violence in recent months.
The arrival of aid convoys to Aleppo is part of the US-Russia ceasefire deal.
Earlier this month, Syrian government forces and their allies captured areas they lost south of the city, re-imposing a siege on its opposition-held eastern neighbourhoods.
More than 2,000 people were killed in 40 days of fighting in the city, including 700 civilians, among them 160 children, according to a Syrian activist group.
Syrian state TV reported Sunday that dozens of residents had left rebel-held areas in Aleppo and were taken to shelters in the government-controlled part of the city.
Also Sunday, Aleppo's governor, Hussein Diab, called on insurgents in the eastern neighbourhoods to turn themselves in, hand over their weapons and take advantage of an amnesty decree issued recently by Syrian president Bashar Assad.
He urged insurgents to halt what he called the bloodshed and destruction and affirmed that all who turned themselves in and surrendered their weapons would be treated well and allowed to return to normal life.
Moscow laid the blame for Sunday's violence squarely on the opposition.
Russian defence ministry spokesman Igor Konashenkov said in a statement that both "terrorists and the opposition" are using the truce to "boost their forces and prepare for renewed hostilities".
Konashenkov says Moscow still has not been able to contact the US-backed opposition to co-ordinate ceasefire efforts despite Washington's assurances. He said the US has not even tried to get the opposition to hold its fire.
Earlier Sunday, Islamic State militants shot down a Syrian warplane as Syrian forces regained ground lost to the extremists following a US-led airstrike that hit government forces the day before, state media said.
Syria called Saturday's US-led strikes on the outskirts of the eastern city of Deir el-Zour a "dangerous and blatant aggression against the Syrian Arab Republic and its army".