Rebels in Syria have announced a big offensive aimed at breaking the government siege of east Aleppo.
A UK-based monitoring group says rebels fired "hundreds" of missiles on western Aleppo, killing at least 15 civilians.
In response, Russia's defence ministry asked permission from President Vladimir Putin to resume air strikes against the rebels after a 10-day pause, but he denied the request.
About 275,000 people have been besieged in the east of the city for months.
Russian and Syrian government planes began bombarding the city in September. More than 2700 people are said to have been killed and injured since then.
Russia suspended its air campaign on 18 October to allow evacuations of sick and wounded people but few have heeded the call to leave.
Russia's defence ministry said continued civilian deaths and an upsurge in rebel activity had prompted its request to resume the strikes.
Kremlin spokesman Dmitry Peskov said Mr Putin did not consider it appropriate to restart the strikes, and said the humanitarian pause should continue.
Meanwhile, US officials have confirmed reports that a Russian fighter jet and a US-led coalition plane were involved in a near miss over Syria on 17 October, coming within half a mile of each other.
The attacks so far were coming from rebels outside Aleppo, but it is understood rebels inside the city will join the offensive.
"All the revolutionary factions, without exception, are participating in the battle," the military spokesman for the Fastaqim faction inside the city said, AP news agency reports.
They included Jabhat Fateh al-Sham, formerly the al-Qaeda-linked Nusra Front, as well as fellow Islamists Ahrar al-Sham.
Rebels fired Grad missiles at al-Nayrab airport, the Syrian Observatory for Human Rights (SOHR) monitoring group said. Up to 115 people were killed or wounded, it added.
They also detonated two car bombs against government positions to the south-west of the city.
It is understood parts of south-western Aleppo have been captured by a group from outside the city.
However, the Syrian government said it had repelled offensives on several fronts around Aleppo by rebel groups as well as militants from the Islamic State militant group.
"Booby-trapped cars, two tanks and a large number of armoured vehicles" were destroyed, the defence ministry said on its website.
The area around the Hmeimim air base near the coastal city of Latakia - the main airport used by the Russian air force - was also reportedly targeted by rebels, the SOHR said.
They hit an airbase and were targeting other Syrian positions in an effort to cripple any response.
But the Syrian army was the least of their problems - by itself, it is relatively weak. The Syrian government is supported by Lebanese, Iranian, Iraqi and Afghan Shia militias, as well as Russian air power - soon to be bolstered by a naval fleet in the Mediterranean.
One of the main rebel factions spearheading this operation is Jabhat Fatah al-Sham, which is affiliated to al-Qaeda. It is the very group the US has tried to untangle from the fighters it supports.
That meant that from a Western perspective at least, the rebels were on their own.
The rebel offensive came as Syria's foreign minister Walid Muallem met his Russian and Iranian counterparts, Sergei Lavrov and Mohammad Javad Zarif, in Moscow.
Mr Lavrov reiterated Moscow's call for moderate rebel groups to disassociate themselves from Jabhat Fateh al-Sham.
He quoted US Secretary of State John Kerry as saying if this did not happen the moderate groups concerned would be considered a legitimate target.
"I think ... the time has come for the logic set out by John Kerry to find some practical shape," he said.
It is the rebels' second attempt to break the Aleppo siege. In August they temporarily opened a corridor between the east and west after the government entirely encircled them for the first time in July.
The city, previously Syria's economic hub, has been divided between the two sides since 2012.