GENEVA: Hundreds of thousands of civilians could be cut off from food supplies if Syrian government forces succeed in their offensive to encircle rebel-held parts of Aleppo, the United Nations said on Tuesday, warning of a massive new flight of refugees.

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.

The assault to surround Aleppo, once Syria's biggest city with 2 million people, amounts to 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.

The United Nations is worried that the government advance could cut off the last link between rebel-held parts of Aleppo and the main Turkish border crossing, which has long served as the lifeline for insurgent-controlled territory.

"If the GoS (government of Syria) and allies sever the last remaining flight route out of eastern Aleppo City 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 in an urgent bulletin.

"If the GoS advances around the city continue, local councils in the city estimate that some 100,000 – 150,000 civilians may flee toward Afrin and the western countryside of Aleppo governorate."

The UN World Food Programme said in a statement it had begun food distribution in the Syrian town of Azaz near the Turkish border for the new wave of people displaced by the fighting.

“The situation is quite volatile and fluid in northern Aleppo with families on the move seeking safety,” said Jakob Kern, WFP’s country director in Syria.

“We are extremely concerned as access and supply routes from the north to eastern Aleppo city and surrounding areas are now cut off, but we are making every effort to get enough food in place for all those in need, bringing it in through the remaining open border crossing point from Turkey.”

The Russian-backed government assault around Aleppo, as well as advances further south, helped torpedo the first peace talks for nearly two years, which collapsed last week before they got under way in earnest.

Moscow turned the momentum in the war in favor of its ally President Bashar al-Assad when it joined the conflict four months ago with a campaign of air strikes against his enemies, many of whom are supported by Arab states, Turkey and the West.

German chancellor Angela Merkel accused Russia this week of bombing civilians, against a UN Security Council resolution Moscow signed up to in December. Russia says it is targeting only Islamist militants.

The complex multi-sided civil war has drawn in outside powers, with the United States leading a separate campaign of air strikes against Islamic State militants who control eastern Syria and northern Iraq.