Air strikes mostly hit the Bustan al-Qasr neighborhood, Zakaria Malhifji of the Aleppo-based Fastaqim rebel group told Reuters.
"There is renewed bombardment and it is heavy," he said.
The Observatory said the death toll from bombing in Bustan al-Qasr, Fardous and other neighborhoods rose to at least 25, with scores of wounded.
At least 50 civilians were killed by strikes on the rebel-held part of the city and nearby villages controlled by insurgents, residents and rescue workers said. In Bustan al Qasr, residents said, the strikes hit a medical center and a children's playground.
The Syrian army, backed by Iranian-backed militias, also said it had consolidated its control of the al Jandoul traffic circle at a major road intersection on the northern outskirts of Aleppo.
Moscow and Damascus reduced air raids in the northern city last week. The Syrian army said that was partly to allow civilians to leave opposition-held eastern neighborhoods.
The Syrian government said rebels holed up in Aleppo can leave with their families if they lay down their arms. Insurgents denounced that offer as a deception.
President Bashar al-Assad seeks the complete recapture of Aleppo, Syria's biggest city before the 5 1/2-year war, which has been divided between government and opposition control for years.
Assad's Russian allies have meanwhile built up its forces in Syria after a brief ceasefire collapsed last month.
Since Russia intervened in the war a year ago, the government's side has gained the upper hand on numerous fronts, including Aleppo, where the opposition-held sector has been completely encircled for weeks.
Insurgents have advanced elsewhere, including in Hama province further south, where they captured a series of towns and villages last month. But in recent days government forces have regained some of that ground.
In the southern city of Deraa, which is split between government and rebel control, insurgent shelling of a school killed at least five people, including children, on Tuesday, the Observatory and state media reported.
Rebels denied they fired at the school. Residents reported the same death toll.
Near the Turkish border, rebels backed by Turkey and a US-led coalition closed in on the Islamic State-held village of Dabiq, the site of an apocalyptic prophesy central to the militant group's ideology.
Free Syrian Army (FSA) rebel groups have been pushing south into Islamic State's territory in an operation backed by Turkey since Aug. 24, and have taken more villages near Dabiq in recent days.
But hundreds of mines planted by the militants were delaying their progress, rebels said. The militants even retook the villages of Ihtimlat and Kafra only hours after the FSA fighters seized them, rebels said.
"They planted along their front lines of defense hundreds of mines," a rebel from the Failaq al Sham group said. Now the goal of FSA forces was to retake the town of Soran, an Islamic State stronghold in the area, before moving on to Dabiq, he said.
UK’s Johnson warns Russians
British Foreign Secretary Boris Johnson warned Russia on Tuesday it risked becoming a pariah nation if it continued to bomb civilian sites in Syria and urged protesters to demonstrate outside the Russian embassy against its alleged war crimes.
"If Russia continues on its current path then I think that great country is in danger of becoming a pariah nation," Johnson told parliament, calling on anti-war protest groups to demonstrate outside the Russian embassy.
"I'd certainly like to see demonstrations outside the Russian embassy," he said, calling on anti-war groups such as the Stop the War coalition, which is backed by the leader of Britain's main opposition Labour Party, Jeremy Corbyn.
Johnson also said the West should do more to put pressure on Russia, which has helped the Syrian government gain the upper hand against rebels on many frontlines in the conflict. He blamed Russia for an attack on an aid convoy in Syria last month.
"We've got to make sure we have innovative ways of getting aid into Aleppo, and as several members have said, we have to step up the pressure on Assad's regime through sanctions and on the Russians through sanctions," he said.
Putin cancels meeting with Hollande
Russian President Vladimir Putin has canceled a visit to Paris next week after President Francois Hollande said he would see him only for talks on Syria - the latest episode in deteriorating relations between Moscow and the West.
French officials have been grappling for ways to put new pressure on Russia after Moscow vetoed a French-drafted United Nations Security Council resolution on Syria. French officials' growing anger over a Russian-backed Syrian government onslaught against rebel-held areas of the city of Aleppo had led them to reconsider whether to host Putin on Oct. 19.
"I made it known to Mr Putin that if he came to Paris, I would not accompany him to any ceremonies, but that I was ready to continue the dialogue on Syria. He decided to postpone the visit," Hollande said at the Council of Europe in Strasbourg.
The Russian president had been scheduled to inaugurate a new Russian Orthodox cathedral and visit a Russian art exhibition in the French capital on Oct. 19.
The Kremlin confirmed Putin's decision, but made no mention of Syria and said he was ready to come to Paris at Hollande's convenience.
While Paris has said it is vital to keep dialogue going with Moscow and not sever relations, events in Syria have damaged their ties as the two countries support opposite sides in the conflict.
Describing Russian air strikes in Syria as "war crimes", Hollande said it was still necessary to talk with Moscow, but only if discussions were "firm, frank," otherwise it would be a "charade."
"With Russia, France has a major disagreement on Syria and the Russian veto of the French resolution at the UN Security Council has prevented the cessation of bombings and enablement of a truce," Hollande said at the Council of Europe.
"I'm ready to meet President Putin if we can advance peace, end the bombings and announce a truce," he said.
France's foreign minister said on Monday his diplomats were working to find a way for the International Criminal Court's prosecutor to launch an investigation into war crimes it says have been committed by Syrian and Russian forces in eastern Aleppo.
Diplomats have also said Paris is leading discussions on whether to impose new European Union sanctions on Russia specifically over Syria, where Moscow backs President Bashar al-Assad in a more than five-year-old civil war.