Regime ally Russia carried out its heaviest strikes in days on Syria's Aleppo on Tuesday, as rebel fire killed at least five schoolchildren in the war-torn country's south.
The raids in Aleppo killed 25 civilians, a monitor said, and caused massive damage in several residential areas of the city's rebel-held east.
Russian President Vladimir Putin meanwhile cancelled a trip to Paris in a row over the violence in Syria, where Moscow is helping President Bashar al-Assad's forces in an operation to recapture all of Aleppo.
Syria's army announced a bid last month to retake the city, which has been divided since mid-2012.
The assault began after the collapse of a short-lived truce negotiated by Washington and Moscow, and has seen the besieged east of the city come under fierce aerial assault.
The army said last Wednesday it would reduce its bombardment, after days of attacks that killed hundreds and destroyed the largest remaining hospital in the east.
But an AFP correspondent and the Syrian Observatory for Human Rights reported renewed heavy bombing on Tuesday.
"This is the heaviest Russian bombardment since the Syrian regime announced it would reduce the bombardment" last week, said Observatory head Rami Abdel Rahman.
The 25 dead, among them four children, were killed in raids in east Aleppo including in the districts of Bustan al-Qasr and Fardos, the Observatory said.
- Children caught in crossfire -
An AFP correspondent in Bustan al-Qasr saw a multi-storey residential building that had been destroyed, its facade sheared off.
White Helmets rescuers removed two lifeless toddlers from the building and wrapped them in sheets.
Footage by the Aleppo Media Centre activist group showed a toddler, blood smeared across her face, lying on a hospital bed.
An elderly man near her wailed in pain as a team of medics bent over him, calling out instructions to the nurses.
The Britain-based Observatory -- which relies on a network of sources inside Syria for its information -- says it determines what planes carried out raids according to their type, location, flight patterns and the munitions involved.
Backed by Russian air raids, government forces have been advancing street by street into rebel-held parts of Aleppo.
More than 300 people, mostly civilians, have been killed by government or Russian fire since the operation began, according to the Observatory.
Rebel forces were also firing on western government-held districts of Aleppo on Tuesday, with state news agency SANA reporting four dead and 14 wounded in rebel bombing of Hamdaniyeh district.
SANA also reported an unidentified number of injuries in a mortar shell attack near the famous Umayyad mosque in the Old City of Damascus.
An AFP correspondent reported intense mortar fire raining down on several neighbourhoods of the capital.
- France-Russia tensions -
Elsewhere, state media said five children were among six people killed in rebel rocket fire on a primary school in the southern city of Daraa.
The Observatory also reported the deaths, saying at least 25 people were wounded and the death toll could rise.
Rebels hold most of Daraa province, but the regional capital is largely controlled by the government.
The assault on Aleppo has sparked international condemnation, with fears for the fate of more than 250,000 civilians trapped inside the east of the city since the government imposed a siege in mid-July.
French President Francois Hollande Sunday described the Aleppo campaign as a war crime, a day after Moscow vetoed a French-drafted UN resolution on a halt to air strikes on the city.
In a sign of escalating tensions, the Kremlin Tuesday said Putin had called off an October 19 visit to inaugurate an Orthodox church in Paris, but was "ready to visit when it is comfortable for President Hollande".
Moscow would "wait for when that comfortable time comes," said Kremlin spokesman Dmitry Peskov.
The French presidency said it had told the Kremlin Hollande would only meet Putin for a "working meeting" on Syria.
British foreign minister Boris Johnson meanwhile called for anti-war campaigners to protest outside the Russian embassy in London.
Johnson said the "wells of outrage are growing exhausted" and anti-war groups were not expressing sufficient outrage over Aleppo.
More than 300,000 people have been killed in Syria since the conflict began in March 2011, and more than half the population has been displaced.