The Syrian army has renewed its heavy shelling of rebel-held areas of Aleppo, the country's commercial capital, anti-government activists have said, forcing the armed opposition to withdraw from at least one neighbourhood.
Intense bombardment rocked the neighbourhoods of Saleheddin, Hanano, Saif al-Dawla, Shaar and Shakur on Thursday, anti-government groups have said.
The army attacks are part of the government's continuing air and ground assaults on Aleppo, which is being defended by armed members of the movement to overthrow Bashar al-Assad, the Syrian president.
The battle for the city has now raged for more than two weeks.
The fighting in Aleppo has focused mainly on the neighbourhood of Salaheddin, where heavy clashes have taken place in recent days between the Free Syrian Army and government forces.
On Thursday morning, FSA commanders said that they were withdrawing from some parts of the neighbourhood, under heavy government fire.
"The FSA has withdrawn from two streets where there has been fighting in recent days," said Hossam Abu Mohammed, commander of the Dara al-Shahbaa Brigade. "The fighters are withdrawing to [nearby] Sukari district, where they are preparing a counter-attack against the army."
"A large number of civilians were killed, as were some 40 rebels," said Abu Mohammed. "Forty buildings have been flattened."
Another FSA commander in Salaheddin confirmed that the rebels are staging a tactical withdrawal.
Red Cross access
"We will open new fronts in Saif al-Dawla and Mashhad districts," to the east of Salaheddin, said the FSA's Wassel Ayub.
Mohamed Saeed, an opposition activist in Aleppo, told Al Jazeera that Syrian forces were using air and artillery fire to assault Salaheddin. He said that the fighting was "the worst" that Aleppo has seen since the uprising began.
In-depth coverage of escalating violence across Syria
Aleppo, the largest city in Syria and its commercial centre, holds great symbolic and strategic importance. Some 40km from the Turkish border, it has been a pillar of government support during the uprising.
An opposition victory there would allow easier access for weapons and fighters from Turkey, where many rebels are based.
Also on Thursday, the Red Cross said it has delivered vital food and medical supplies to Aleppo, the first time one of its aid convoys managed to enter the besieged city since fighting intensified a few weeks ago, a spokesman said.
A convoy of trucks brought one month's food supplies for 12,500 people and wound-dressing material for up to 1,000 injured, depending on the severity of their wounds, the International Committee of the Red Cross said in a statement.
"Despite the difficult security conditions, the convoy managed to get inside the city," Hicham Hassan, ICRC spokesman, told the Reuters news agency.
Refugee boat intercepted
Until Thursday, it had been unable to send supplies to Aleppo as its minimum security requirements were not met, despite a mounting need for humanitarian aid, Hassan said.
In a seperate development, a boat carrying 160 people who are believed to have fled fighting in Syria, was intercepted by the Italian police off the coast of Calabria.
The vessel was carrying 68 men, 25 women and 65 minors.
The migrants have been transported to the Isola di Capo Rizzuto temporary detention camp for identification and two Turkish nationals were arrested.
Thousands of Syrians are continuing to flee the escalating violence in their country, seeking safety in neighbouring countries.
More than 200,000 Syrians have fled to neighbouring Jordan, Lebanon and Turkey and hundreds of thousands more are displaced within Syria.