Syrian air strikes and shelling killed 25 civilians in eastern districts of Aleppo on Thursday, a monitor said, on the third day of a wide-ranging regime assault on rebel-held areas.
The bombardment hit at least six rebel-held neighborhoods, said the Syrian Observatory for Human Rights.
At least 65 civilians have been killed in east Aleppo since the start of a regime offensive on Tuesday, said the monitoring group.
On Wednesday, at least 21 people were killed, five of them children, when Syrian airstrikes damaged a children's hospital, a blood bank, and a school.
The renewed bombardment has shattered a month of relative calm in the devastated east of the divided northern city.
An AFP correspondent in the eastern districts said explosions from barrel bombs dropped by aircraft had been heard since 10:00 am (0800 GMT).
Barrel bombs are crude weapons -- containers packed with explosives and scrap metal that are typically dropped from helicopters and which the Syrian has repeatedly denied using.
One of the air strikes on Thursday targeted a civil defense center in the Bab al-Nayrab neighborhood with no reported casualties, the AFP correspondent said.
The Observatory said rebels responded with shelling of the city's government-controlled western neighborhoods.
Once Syria's economic powerhouse, Aleppo has been roughly divided into a regime-controlled west and a rebel-held east since 2012.
No aid has entered the city's eastern neighborhoods since government troops surrounded them in mid-July, and humanitarian organizations said this week food aid stockpiled there had all but run out.
The recent bombardment has ended a period of relative respite in east Aleppo, where regime ally Russia halted air strikes on October 18 ahead of a series of brief ceasefires.
Moscow said on Wednesday it had not carried out any raids on the city since that date.
On Thursday, however, its defense ministry said it had bombed jihadist groups in Syria using cruise missiles launched from bombers deployed from Russian territory.
It said Thursday's strikes targeted the Islamic State (ISIS) group and former Al-Qaeda affiliate Fateh al-Sham Front, but it did not specify where they had been carried out.
Russia said this week it was launching a major operation against ISIS and Fateh al-Sham Front, including in the northern province of Idlib.
Syria's war has displaced more than half the population and killed over 300,000 people since it started in March 2011 with anti-government protests.
Russia intervened in Syria in September 2015 in support of President Bashar Al-Assad's regime.
AFP contributed to this report.