Syria's military has said its joint airstrikes with Russia have killed at least 400 insurgents over the past 24 hours - as Vladimir Putin reaffirmed his support for Syrian President Bashar al Assad.
The strikes are being carried out after insurgents led by jihadi group Hayat Tahrir al Sham and including Turkey-backed fighters launched a two-pronged attack on Aleppo, Syria's second largest city, on Saturday.
The rebels then moved into the countryside around the provinces of Idlib and neighbouring Hama.
Syrian and Russian airstrikes have mainly been carried out on rebel positions in those two provinces, Syria's military said.
Syria's opposition-run White Helmets rescue service said on Monday that at least 25 people, including 10 children, have been killed in airstrikes carried out by the Syrian government and Russia.
It comes as Mr Putin and Iranian President Masoud Pezeshkian spoke in a phone call about the "escalating situation" in Syria, the Kremlin said.
The pair, who are both longtime allies of the Syrian president, discussed "large-scale aggression" by rebel fighters which they viewed "as an attempt to undermine the sovereignty, political, and socio-economic stability of the Syrian state", a statement read.
Mr Putin and Mr Pezeshkian expressed their "unconditional support" for the Syrian government, it added.
Meanwhile, Kremlin spokesman Dmitry Peskov told reporters on Monday: "We, of course, continue to support (Syrian president) Bashar Assad, we continue our contacts at the appropriate level and analyse the situation.
"A position will be formed regarding what is needed to stabilise the situation."
Separately, Mr Pezeshkian told Mr Assad in a phone call that Tehran is willing to provide all the support needed to push back the insurgency.
It came as Syria's military said on Monday that government forces were mobilising to encircle rebels in the countryside of Aleppo, Hama and Idlib.
Meanwhile, two Syrian army sources told Reuters that Iran-backed militant groups had entered Syria from Iraq.
They added that the Iraqi militants were heading north to bolster Syrian army forces battling rebels.
According to the Syrian Observatory for Human Rights, a UK-based opposition war monitor, some 200 Iraqi militiamen riding on pickups crossed into Syria overnight through the strategic Bou Kamal crossing.
They were expected to deploy in Aleppo to support the Syrian army's pushback against the insurgents, the monitor said.
Read more:
Who are the Syrian rebels?
Analysis: Why the significance of Syria's rebel offensive is immense
The push by the rebels in the north of the country is among their strongest in years and raised the prospect of another violent front reopening in the Middle East, at a time when US-backed Israel is fighting Hamas in Gaza and Hezbollah in Lebanon - both Iran-allied groups
It also risks drawing Russia and Turkey - each with its own interests to protect in Syria - into direct confrontation.
The advance by the insurgents is also huge embarrassment for Mr Assad, and it comes at a time when his allies - Iran and the groups it backs, and Russia - are preoccupied with their own conflicts.
(c) Sky News 2024: Syria says its strikes with Russia have killed at least 400 insurgents over past 24 hours