The Governments of the United States, France, Germany, and the United Kingdom, on Sunday issued a joint statement on the current situation in Syria.
“We are closely monitoring developments in Syria and urge de-escalation by all parties and the protection of civilians and infrastructure to prevent further displacement and disruption of humanitarian access,” the four countries said.
“The current escalation only underscores the urgent need for a Syrian-led political solution to the conflict, in line with UNSCR 2254,” they added.
The announcement follows an offensive against the Syrian army by an Islamist-led rebel alliance which was launched last Wednesday, with Assad’s forces losing control of the city of Aleppo on Sunday, according to a war monitor.
On Friday, rebels had managed to penetrate parts of Aleppo and, on Saturday, moved into the center of Aleppo.
Assad has remained unfazed, stressing his country’s resolve "in the face of all terrorists and their backers" in a conversation with UAE President Sheikh Mohamed bin Zayed Al Nahyan.
Russia announced on Sunday that it is assisting the Syrian army in fending off the rebel forces across three northern provinces.
"The Syrian Arab Army, with the assistance of the Russian Aerospace Forces, is continuing its operation to repel terrorist aggression in the provinces of Idlib, Hama and Aleppo," the Russian military said in a briefing on its website.
"Over the past day, missile and bombing strikes were carried out on places where militants and equipment were gathered," the statement added, though it did not specify the locations or identify those responsible for the strikes.
The Russian military claimed that at least "320 militants were destroyed."
Russia has been conducting air strikes in Syria since September 2015, in support of Assad, its close ally.
While Russia insisted it was attacking targets associated with the Islamic State (ISIS) terrorist organization, Western officials claimed Moscow was also targeting so-called "moderate" rebel groups fighting ISIS.