Unidentified jets on Sunday resumed pounding rebel-held areas in Aleppo, ending a lull of hours in Syria's divided northern city, a monitoring group reported.
The Syrian Observatory for Human Rights added that opposition areas in eastern Aleppo were the target of air bombardment, causing unspecified casualties.
Islamist rebels meanwhile fired rockets at the regime-held towns of Nubl and al-Zahara on the northern outskirts of Aleppo, the observatory added, without giving casualties.
At least 14 civilians, including two children, have been killed in airstrikes by government warplanes and those of allied Russia on rebel areas in Aleppo overnight, the watchdog said.
Earlier on Sunday, activists reported a lull in fighting in Aleppo that has seen intense airstrikes since a truce, brokered by the United States and Russia, collapsed Monday night.
UN Secretary General Ban Ki-moon called the attacks, which killed dozens, the "most sustained and intense bombardment" since the Syrian crisis began in 2011.
Opposition fighters recaptured Handarat, a refugee camp on the northern edge of Aleppo, on Sunday, a day after they lost it to regime forces, the observatory said.
Handarat is located near Castello Road, a major supply route to opposition areas in eastern Aleppo.
The surge in violence has worsened the situation inside eastern Aleppo, which has been besieged by government forces since July 17.
The siege has resulted in an acute shortage of food supplies and fuel, according to the observatory, which relies on a network of activists inside Syria.
The UN Security Council was to meet later on Sunday in New York on the situation in Aleppo.
A main opposition grouping said that the military campaign in Aleppo renders a peace process aimed at ending Syria's conflict "futile and senseless."
The West-backed Syrian National Coalition demanded that Russia no longer act as a co-sponsor of negotiations with Damascus.
"The Russian side ... is a partner to the regime in its crimes against our people," the coalition said in a statement.
In September 2015, Russia started an air campaign in Syria in support of President Bashar al-Assad, saying it was targeting extremists there.
In April, indirect peace talks between the Syrian government and the opposition in Geneva fizzled out due to a spike in violence in the country, mainly Aleppo.
Once Syria's commercial hub, Aleppo has been divided between government forces in the west and rebels in the east since fighting erupted for control of the city in mid-2012.