Rebels have bombarded government-held areas of Aleppo with rockets, killing 19 people and hitting a hospital, while also launching a ground assault on army-held positions of the divided city, the Syrian Observatory for Human Rights said.
The Syrian army said insurgents had launched a widespread assault on Tuesday and that it was responding.
State-run Syrian news channel Ikhbariya said three women were killed and 17 more people wounded at the al-Dabit maternity clinic.
The army statement said the attack was at "a time when international and local efforts are being made to shore up the (cessation of hostilities agreement) and to implement ... calm in Aleppo".
The Observatory said the hospital had been heavily damaged.
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In rebel-held parts of Aleppo, the Observatory said there had been three air strikes, citing information of an unconfirmed number of people killed.
The Observatory said 279 civilians have been killed in Aleppo by bombardments since April 22, 155 of whom were killed in opposition-held areas, and 124 in government-held districts.
The ground assault focused on the Jamiat al-Zahraa area of the city, where insurgent groups detonated tunnels and took a few buildings before advances were checked by the arrival of reinforcements on the government side, the Observatory said.
