The UN's refugee agency has warned that thousands of people are still trapped in Syria's economic hub, Aleppo, as the army offensive against rebel strongholds continued for a fourth day.

Activists in the city say supplies of food and cooking gas are running low.

Fresh clashes and attacks by helicopter gunships have been reported, but rebels say they have held on to the key district of Salah al-Din.

The UN says 200,000 people have fled the fighting in the city.

But Melissa Fleming, a spokeswoman for the UNHCR refugee agency, said many more had been unable to leave because they did not have the means to make the journey, or felt it would be too dangerous.

An estimated 15,000-16,600 frightened residents have sought refuge in schools, mosques and public buildings in the city, she said.

"There are 32 schools in Aleppo that we've identified and in each of those schools, we have heard, or we have witnessed, that 250-300 people were packed inside, many of these families, kids.

"And then in university dormitories, there are a total of some 7,000 people hoping to seek safety from the continuing shelling."

Mohammed Saeed, an activist living in the city, told the Associated Press that the situation for residents was "very bad".

"There is not enough food and people are trying to leave. We really need support from the outside. There is random shelling against civilians," he said.

"The city has pretty much run out of cooking gas, so people are cooking on open flames or with electricity, which cuts out a lot."

There are also reports that citizens remaining in the city face long queues for bread.

'Heavy losses'
On Tuesday, there were fresh reports of clashes and attacks by helicopter gunships in Aleppo as the Syrian army offensive entered its fourth day.

Activists in the city say supplies of food and cooking gas are running low.

Fresh clashes and attacks by helicopter gunships have been reported, but rebels say they have held on to the key district of Salah al-Din.

The UN says 200,000 people have fled the fighting in the city.

But Melissa Fleming, a spokeswoman for the UNHCR refugee agency, said many more had been unable to leave because they did not have the means to make the journey, or felt it would be too dangerous.

An estimated 15,000-16,600 frightened residents have sought refuge in schools, mosques and public buildings in the city, she said.

"There are 32 schools in Aleppo that we've identified and in each of those schools, we have heard, or we have witnessed, that 250-300 people were packed inside, many of these families, kids.

"And then in university dormitories, there are a total of some 7,000 people hoping to seek safety from the continuing shelling."

Mohammed Saeed, an activist living in the city, told the Associated Press that the situation for residents was "very bad".

"There is not enough food and people are trying to leave. We really need support from the outside. There is random shelling against civilians," he said.

"The city has pretty much run out of cooking gas, so people are cooking on open flames or with electricity, which cuts out a lot."

There are also reports that citizens remaining in the city face long queues for bread.

'Heavy losses'
On Tuesday, there were fresh reports of clashes and attacks by helicopter gunships in Aleppo as the Syrian army offensive entered its fourth day.

The BBC's Ian Pannell, who has just left Aleppo, says knowing who is firing where is impossible, as gunfire echoes around the city.

State television completely ignored events in Aleppo in its early morning news programmes on Tuesday, the BBC's Jim Muir in Beirut says, but later bulletins carried reports of Syrian forces inflicting heavy losses on ''terrorist groups'' in Aleppo districts.

Correspondents say neither side can afford to lose Aleppo, Syria's largest city and its industrial and financial centre.

Until this month it had been spared the bloodshed seen in other cities since the uprising began in March 2011.

Border security
Meanwhile the US and Turkey have agreed to step up efforts to achieve "political transition" in Syria that would include the departure of President Bashar al-Assad, the White House said.

In a phone call on Monday, US President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan agreed to help the growing numbers of refugees - both inside Syria and in neighbouring countries.

Turkey has been a staunch critic of President Assad and has given refuge to large numbers of army defectors.

Thousands of Syrian refugees are also living in refugee camps on the Turkish side of the countries' long border.