Men attempt to identify bodies after airstrikes by pro-Syrian government forces in the rebel held al-Saleheen neighborhood of Aleppo.

More than 500 people, including dozens of civilians, have been killed since a major Russian-backed regime offensive in Syria’s Aleppo province began this month, a monitor said on Wednesday.

The Syrian Observatory for Human Rights, which relies on a network of sources for its figures, said the toll of 506 included 23 children killed in Russian air strikes on Aleppo city and its surroundings since the operation was launched on February 1.

300,000 in Aleppo seige

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.

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