Syrian groups fighting against the regime took control of the outskirts of Aleppo after triggering two car bombs. The country's military says in a statement that it is fighting to hold back the onslaught, but according to the UK-based conflict monitor Syrian Human Rights Observatory (SOHR), the groups have taken control of parts of the city.
They "control five neighborhoods", says SOHR chief Rami Abdel Rahman, adding that they "advanced without significant resistance from regime forces".
According to the government in Damascus, military reinforcements arrived in Aleppo on Friday.
Fifty towns captured
Russia reports that its air forces attacked the groups' personnel, equipment, and warehouses in Syria on Friday evening, according to state-run Russian news agencies.
Turkey had called for a stop to attacks on Syrian Idlib, where Syrian and Russian warplanes reportedly carried out airstrikes earlier.
Earlier on Friday, it was reported that the groups had captured around fifty government-controlled towns and villages, according to SOHR.
The battles in northwestern Syria are primarily between the Syrian army and the extremist group Hayat Tahrir al-Sham (HTS). The offensive began on Wednesday and the rebel forces have since advanced rapidly.
Iran promises continued support
The battles are among the deadliest in several years. SOHR reports that at least 277 people have been killed. Most are fighters, but over 20 civilians have also been killed, mostly in Russian airstrikes. A general in Iran's Revolutionary Guard is also reported to be among the dead.
There are also reports of artillery fire hitting a university area in Aleppo, causing panic. Government-controlled sources report that at least four civilians were killed.
On Friday, the Russian Kremlin urges Syria's regime to immediately "restore order" around Aleppo. Iran's Foreign Minister Abbas Araghchi promises "continued support for the government, the country, and Syria's military".
Syria's President Bashar al-Assad remains in power after the devastating civil war, which was triggered by the Arab Spring in 2011, has ravaged large parts of the country. al-Assad has continued support from Russia, Iran, and Shia militias in the region. Currently, however, his allies are largely tied up in other wars and conflicts.
The wars just west, in Gaza and Lebanon, have in a way spilled over into Syria as Israel has stepped up airstrikes against targets in Syria.
The Assad regime has limited control over Syrian territory, as Kurdish forces maintain self-rule in the northeast and another collection of rebel forces holds areas in the north and northwest, largely with support from Turkey.
Aleppo was Syria's largest city before the outbreak of war in 2011, but in a long and bloody siege, large parts of it were reduced to ruins by indiscriminate Russian airstrikes. In 2016, government forces regained control of the city, which marked a significant turning point in the war.