Iranian-backed militias have entered Syria overnight from Iraq in a bid to help the Syrian army crush an insurgency in the north of the country.
The reinforcements come as Syrian forces launched airstrikes on Sunday on the northern city of Aleppo where insurgents have seized control.
Iran has pledged to help its ally, Syrian dictator Bashar al-Assad, combat the offensive, with dozens of Iranian-backed Iraqi Hashd al Shaabi fighters crossing into Syria by a military route, Reuters reported.
Insurgents led by jihadi group Hayat Tahrir al-Sham launched a two-pronged attack on Aleppo and the countryside around Idlib on Wednesday, before moving toward neighboring Hama province.
Fighting surged on Sunday after government troops attempted to create a “strong defensive line” in northern Hama.
Jets also pounded Idlib and Aleppo, killing at least 25 people, according to the Syrian civil defence group that operates in opposition-held areas.
The insurgents announced their offensive on Wednesday, just as a ceasefire started between Hezbollah and Israel that raised hope tensions in the region might be calming.
The surprise offensive is hugely embarrassing for al-Assad, and comes when key allies Iran and Russia are preoccupied with their own conflicts.
Arab leaders, including Jordan's King Abdullah II and United Arab Emirates President Sheikh Mohammed bin Zayed Al Nahyan, have also called al-Assad to express their solidarity.
Western governments have urged de-escalation by all parties and protection of civilians and infrastructure "to prevent further displacement and disruption of humanitarian access."
The statement on Sunday by the US, UK, French and German governments said they were watching the situation closely and that the escalation underscored the urgent need for a political solution.
The insurgents took over most of Aleppo on Saturday, seizing key infrastructure such as the main water pumping station for the city.
Rebel commander Colonel Hassan Abdulghani claimed the insurgents had also advanced in the countryside around Idlib, putting all of the province of the same name under their control.
The insurgents have vowed to push all the way into Damascus, but life in the Syrian capital remained normal with no signs of panic, Reuters reported.
In southeastern Aleppo, however, the main road out of the city was gridlocked as people fled the fighting, and gas stations in the area were short on fuel.
The group leading the rebel advance is designated a terrorist organization by the US.
National Security Advisor Jake Sullivan said Washington has "real concerns about the designs and objectives of that organization."
“At the same time, of course, we don't cry over the fact that the Assad government, backed by Russia, Iran and Hezbollah, are facing certain kinds of pressure," he added.
The 2016 battle for Aleppo was a turning point in the war between Syrian government forces and rebel fighters after 2011 protests against Assad's rule turned into an all-out war.
After appearing to be losing control of the country to the rebels, the Aleppo battle secured al-Assad's hold on strategic areas of Syria, with opposition factions and their foreign backers controlling areas on the periphery.
