Heavy bombing pummelled rebel-held parts of Aleppo on Monday, an AFP correspondent in the city said, less than two hours after Syria's army announced the end of a week-long truce.
Air strikes hit five eastern neighbourhoods of the battleground city, while heavy shelling was heard coming from Aleppo's southern outskirts.
Ambulances with wailing sirens zipped through the eastern half of the divided city, the correspondent said, describing the bombardment as "non-stop."
The Syrian Observatory for Human Rights, a Britain-based monitoring group, also reported that shells and rockets were raining down on the northern city.
Syria's armed forces announced an end to the week-long truce on Monday evening, accusing rebel groups of violating the ceasefire more than 300 times.
Under the US-Russia deal, fighting would halt across Syria and humanitarian aid would reach desperate civilians -- particularly in devastated eastern Aleppo.
AFP's correspondents in the city had reported calm over the first few days of the truce, but violence escalated sharply at the weekend.
More than 300,000 people have been killed since Syria's conflict erupted in March 2011; AFP reported.
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