Hundreds of thousands of Syrian civilians in opposition-held areas in Aleppo city are under threat of being cut off from basic food supplies amid expectations of a looming siege by government forces, the United Nations has warned.

The UN said on Tuesday that it is worried government advances could block 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 those areas in Syria.

"It would leave up to 300,000 people, still residing in the city, cut off from humanitarian aid unless...access could be negotiated," the UN 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 to 150,000 civilians may flee," it said.

Aleppo was once Syria's biggest city and home to two million people.

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

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Locals have told Al Jazeera about their fears of a possible siege and the harsh effects of the conflict on food and fuel prices in the city.

A woman, who asked to be called Om Steif, said: "The people here in Aleppo are scared of the coming siege.

"What will happen to the cost of living to citizens like me? How can they afford [to buy] heating fuel when the prices keep doubling and quadrupling every day. How will we face this?"

Zaid Muhammad, a volunteer for Kesh Malek - a Syrian activist group in opposition-held districts of Aleppo, told Al Jazeera how Russian air strikes backing government forces have been "terrorising" civilians daily.

"For seven or eight hours a day they (warplanes) are invading the skies and terrorising the people psychologically," he said

Meanhwhile, the UN's refugee agency has called on Turkey to open the border to tens of thousands of Syrian refugees fleeing the government offensive in Aleppo province, who are stranded near the Bab al-Salameh crossing.

In an interview with Al Jazeera on Tuesday, UNHCR spokesman William Spindler praised Ankara for allowing in a number of wounded refugees and for providing humanitarian assistance to those on the Syrian side of the crossing.

But he said the Turkish government needs to "extend the opening of the border to others in need of protection and fleeing danger", in accordance with its obligations under international law.

Spindler said that his agency acknowledges the fact that Turkey is already hosting 2.5 million refugees, which has inflicted a "huge strain" on the country's economy, and called on the international community to assist Ankara in handling the burgeoning crisis.

"This is clearly an international crisis and we all have an obligation to assist," he said.

Al Jazeera's Zeina Khodr, reporting from the bordering Turkish city of Gaziantep, described a "worsening humanitarian tragedy on the ground" in Aleppo governorate, as refugee camps overflow and the Syrian government onslaught of the city intensifies.

"There are thousands of others who are stuck in the town of Azaz, which is just a few kilometres from the Bab al-Salameh crossing," she said.

"Already hundreds of families are moving from Azaz, because according to Doctors Without Borders there is no more space, the camps can no longer absorb more people and people are sleeping out in the open.

"These people are getting on buses and heading to the western side of Aleppo after receiving safe passage from the YPG (Kurdish People's Protection Units).