Refugees who fled bombing in Aleppo wait at the Bab al-Salam crossing, opposite the Turkish province of Kilis.

Hundreds of thousands of civilians could be cut off from food if Syrian government forces encircle rebel-held parts of Aleppo, the United Nations said on Tuesday, warning of a new exodus of refugees fleeing a Russian-backed assault.

The army aims to secure the border with Turkey and recover control of Aleppo, a senior adviser to Syrian President Bashar al-Assad told Reuters, adding that she did not expect diplomacy to succeed while foreign states maintain support for insurgents.

Syrian government forces, backed by Russian air strikes and Iranian and Lebanese Hezbollah fighters, have launched a major offensive in the countryside around Aleppo, which has been divided between government and rebel control for years.

It marks one of the most important shifts of momentum in the five-year civil war that has killed 250,000 people and already driven 11 million from their homes.

Since last week, fighting has already wrecked the first attempt at peace talks for two years and led rebel fighters to speak about losing their northern power base altogether.

The U.N. is worried the government advance could cut off the last link for civilians in rebel-held parts of Aleppo with the main Turkish border crossing, which has long served as the lifeline for insurgent-controlled territory.

“It would leave up to 300,000 people, still residing in the city, cut off from humanitarian aid unless cross-line access could be negotiated,” the United Nations Office for Coordination of Humanitarian Affairs said.

If government advances around the city continue, it said, “Local councils in the city estimate that some 100,000 – 150,000 civilians may flee”. Aleppo was once Syria’s biggest city, home to 2 million people.

Air strikes continued on Tal Rifaat, Anadan and other towns in the Aleppo countryside, the Syrian Observatory for Human Rights, which monitors the war, and activists said, adding that they were almost certainly from Russian planes.

Independent Doctors’ Association official Mahmoud Mustafa said at least two people had been killed and 30 wounded in air strikes near Tal Rifaat and villages near Azaz. The death toll could be higher since his figures referred only to victims brought to the organization’s hospital on the Syrian side of the Oncupinar border crossing with Turkey.

Related articles across the web

• Russian airstrikes in Syria's Aleppo kill 4, injure 60
• Thousands of Syrians Flee Aleppo to Escape Russian Airstrikes
• Is Syrian President Bashar Al Assad Finally Winning?
• Thousands of Syrians wait at closed Turkey border after Russian airstrikes on Aleppo
• An exasperated John Kerry: 'What do you want me to do, go to war with the Russians?!'
• Mass Exodus From Aleppo As Attack Increases
• Thousands Of Syrians Flee Toward Turkey Amid Government Offensive
• Turkey PM: 'Starvation siege is being inflicted on Syria'
• Number of Syrian refugees fleeing Aleppo doubles