UN Secretary-General Antonio Guterres is "alarmed" by the escalation of violence in Syria and is calling for an immediate end to fighting, his spokesman said Monday.
"All parties must do their utmost to protect civilians and civilian objects, including by allowing safe passage to civilians who are fleeing hostilities," spokesman Stephane Dujarric said in a statement.
"Syrians have endured the conflict for nearly 14 years. They deserve a political horizon that will deliver a peaceful future, not more bloodshed," he added.
Syria has been at war since President Bashar al-Assad cracked down on democracy protests in 2011, with subsequent fighting that has involved foreign powers and jihadists leaving some 500,000 people dead.
The conflict had been mostly dormant with Assad back in control of much of the country until last week, when the Islamist-led rebel alliance began its offensive.
Syria's military and its ally Russia have responded with deadly air raids on areas under rebel control.
UN peacekeeping operations in the country have been "largely suspended" across Aleppo, Idlib and Hama due to security concerns, Dujarric said.
"This has led to severe disruption in people's ability to access life-saving assistance," he said, adding that the United Nations remains committed to delivering humanitarian aid.
Dujarric warned that "the presence of unburied bodies and lack of drinking water" in Syria threatens public health and said that damage to Aleppo's university hospital had left hundreds of patients without care.
"Syria is also already one of the world's largest humanitarian crises, with 16.7 million human beings in need of assistance and over 7 million people internally displaced," he said.
"More than half a million people have also fled from Lebanon to Syria in recent weeks, and winter conditions will only make the needs in the coming weeks even more acute."
