ALEPPO -  Regime ally Russia carried out its heaviest strikes in days on Syria's Aleppo Tuesday, as at least five children were killed in rebel fire on a school in the war-torn country's south.

The raids killed 16 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 planned 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.

The announcement from the Kremlin came a day after French President Francois Hollande said Syrian forces had committed a "war crime" in the battered city of Aleppo with the support of Russian air strikes.

Putin had been due in Paris on October 19 to inaugurate a new Russian Orthodox church near the Eiffel Tower, but Hollande had insisted his Russian counterpart also took part in talks with him about Syria.

The French president had admitted he was agonising over whether to meet Putin, but the Kremlin on Tuesday called off the visit.

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 bombing that killed hundreds and destroyed the largest remaining hospital in the rebel-held east. But an AFP correspondent and the Britain-based 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, Observatory head Rami Abdel Rahman said.

The 16 dead, among them four children, were killed in raids in the Bustan al-Qasr and Fardos neighbourhoods, the Observatory said.

An AFP correspondent in Bustan al-Qasr saw a multi-storey residential building that had been destroyed, its facade sheared off in the air attack.

Members of the White Helmets rescue force pulled two lifeless toddlers from the building and wrapped them in white sheets. Footage by the Aleppo Media Centre activist group showed a toddler, blood smeared across her face, lying on a hospital bed.

An older man near her is wailing in pain as a team of medics bends 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.

At least 290 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 on Tuesday.

An AFP correspondent in Damascus said later that there was intense mortar fire raining down on several neighbourhoods of the capital.

Elsewhere in Syria, 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 because a number of the wounded were in critical condition. Rebel forces hold most of Daraa province, but the provincial 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 on Sunday described the campaign in the city as a war crime, a day after Moscow vetoed a French-drafted UN resolution on a halt to air strikes on Aleppo.

In a sign of escalating tensions, the Kremlin on 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 that Hollande would only meet Putin for a "working meeting" on Syria.

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.

Meanwhile, Amnesty International on Tuesday welcomed Jordan's move to allow aid deliveries to thousands of Syrians trapped on its desert border, but called for a long-term solution to the crisis.

More than 70,000 Syrians are trapped in "hellish conditions" in a remote, arid strip of no-man's land on the Syrian side of the border, the rights group said.

Jordan closed the border, halting aid deliveries to a makeshift camp, after a bombing claimed by the Islamic State group killed seven Jordanian soldiers in June.

Since then the kingdom has only allowed a single delivery of aid to the refugees, in August.

But it said on Monday it would allow further deliveries by crane across the border in the coming weeks.

Amnesty said the announcement was "a long-awaited glimmer of hope that should be followed by a sustainable, long-term solution".

"News that humanitarian assistance will be resumed to tens of thousands of refugees... comes as a welcome relief," said Audrey Gaughran, Amnesty's director of global issues and research.

"However, we are worried by reports that aid will be lobbed across the border by crane or that refugees will be coerced... to move to areas where they may be at risk of attack in order to receive it."

She called for unfettered humanitarian access and for the refugees to be allowed onto Jordanian territory.

Jordan closed its entire desert border with Syria and Iraq, after the bombing near the Rukban crossing on June 21.

Jordanian officials said the bomber had come from a camp just across the border.

Jordan says it is hosting over 1.4 million Syrians on its territory, of which 630,000 are registered with the United Nations.

The kingdom has repeatedly said it is not receiving enough international help to share the burden.