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.

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 United Nations 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.

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A girl mourns the loss of her relatives after airstrikes by pro-Syrian government forces in the rebel held al-Saleheen neighbourhood of Aleppo, Syria, on Monday. (Abdalrhman Ismail/Reuters)

"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 government advances around the city continue, it said, "local councils in the city estimate that some 100,000-150,000 civilians may flee".

Turkey's border closed

Turkey, already home to 2.5 million Syrians, the world's biggest refugee population, has so far kept its frontier closed to the latest wave of displaced, making it more difficult to reach them with urgently needed aid. The United Nations urged Ankara on Tuesday to open the border and has called on other countries to assist Turkey with aid.

Turkish Prime Minister Ahmet Davutoglu said as many as a million Syrian refugees could arrive if the Russian-Syrian campaign continues.

Fifty thousand people had reached Turkey's borders in the latest wave. Ankara had admitted 10,000 so far and would allow in others in a "controlled fashion", he said.

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."

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Boys run near a hole in the ground after airstrikes by pro-Syrian government forces in the al-Sakhour neighbourhood of Aleppo. (Abdalrhman Ismail/Reuters)

Competing report

Another report says more than one million Syrians are trapped in besieged areas.

The new Siege Watch report, issued Tuesday by the Netherlands-based non-profit PAX and the Washington-based Syria Institute, says 1.09 million people are living in 46 besieged communities in Syria, far more than the 18 listed by the UN.

It says most are besieged by the Syrian government in the suburbs of Damascus, the capital, and Homs. In the eastern city of Deir el-Zour, about 200,000 people are besieged by both ISIS and the Syrian government. The report lists the two communities besieged by armed opposition groups.

"Electricity and running water are usually cut off, and there is limited (if any) access to food, fuel, and medical care," the report says. Deaths have been reported from malnutrition, disease, hypothermia and poisoning while scavenging for food. Some communities have been besieged for months or years.

The estimates are based largely on information provided by local contacts in the communities, including local councils, medical workers and citizen journalists.

"Many remain unaware of the extent of the crisis, and the international response has been muted as a result," the Siege Watch report says.

Russian-backed assault

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

Moscow turned the momentum in the war in favour 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 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 ISIS militants who control eastern Syria and northern Iraq.

A suicide bomber drove his car into a police officers' club in a residential quarter in central Damascus, blowing himself up and killing at least 10 people and destroying a number of cars, a Syrian interior ministry statement said.

with files from the Associated Press