"I am extremely concerned by the impact of shelling and use of tanks and other heavy weapons on people in Aleppo," Valerie Amos, the top U.N. official for humanitarian affairs, said in a statement late Sunday. "Many people have sought temporary shelter in schools and other public buildings in safer areas," she added. "They urgently need food, mattresses and blankets, hygiene supplies and drinking water."
Amos said U.N. agencies and the Syrian Red Crescent are working together on supplying those affected by the fighting with blankets and humanitarian supplies, but many remain out of their reach because of the violence.
"It is not known how many people remain trapped in places where fighting continues today," she warned. Aleppo is Syria's largest city and commercial hub with about 3 million inhabitants.
Aleppo is some 50 kilometers (30 miles) away from the Turkish border and some of those fleeing the city are headed for Turkey, where tens of thousands of Syrians have already found refuge during the 17-month uprising against authoritarian President Bashar Assad's rule.
Turkey is deploying more troops to the border, sending tanks, armored combat vehicles, more missile launchers and infantry troops, the state-run Anadolu agency reported.
Greece has responded by quadrupling the number of guards on its borders with Turkey out of fear of a potential influx of Syrian refugees.
Those fleeing Aleppo have described to The Associated Press incessant shelling, shortages of food and gasoline and soaring black market prices for everyday staples. In online videos they can be seen scurrying through streets against a backdrop of gunfire and climbing onto any form of transportation available to escape, including trucks, cars and even heavily laden motorcycles.
"Dozens of families are packing their belongings and leaving in cars and trucks," said an activist in a village near Aleppo, who declined to give his name for security reasons. "They are only taking light possessions that they can carry, like a few clothes, some valuables and that's it."
"I saw cars with eight, nine people packed in them fleeing the bombing," he added. He said rebels had seized a nearby checkpoint early in the morning and captured several tanks. The regime responded by shelling the rural area just northwest of the city. "Entire families are leaving."
Videos of the attack on the checkpoint posted Monday in the Internet show fierce exchanges of fire in the early morning and then later, victorious rebels hauling out boxes of ammunition and taking heavy machine guns for the fight in Aleppo.
The activist said refugees have fled to nearby villages where they stay with relatives or find shelter in local schools. Some however, are making the trip to the Turkish border.
Syrian state media had reported the army had "purged" Aleppo's southwestern neighborhood of Salaheddine and inflicted "great losses" upon the rebels in one of the first districts they took control of in their bid to seize the city.
Activists, however, disputed these claims and just described another day of fierce shelling of certain areas, backed up by the occasional foray on the ground. The assault has knocked down power lines and the neighborhood has been without electricity since the morning.
Gen. Babacar Gaye, the head of the United Nations Observer Mission in Syria expressed concerned over the ongoing violence in Aleppo, noting that "helicopters, tanks and artillery are being used."
Gaye visited the town of Rastan, just outside the city of Homs in central Syria, which has witnessed heavy fighting. Footage from his trip showed a devastated town with burnt out tanks by the side of the road and many destroyed buildings.
"We will be monitoring the level of violence and the use of heavy weapons in Syria. We will also be assessing if there is readiness and, if possible, progress for local confidence-building measures and national dialogue," said Gaye, whose mission is to monitor a cease-fire that has never been observed by either side.
U.S. Defense Secretary Leon Panetta said late Sunday that the use of heavy weapons, particularly helicopters, is just another nail in President Bashar Assad's coffin. He spoke during a stopover in Tunisia as he kicked off a Mideast tour expected to focus heavily on the unfolding crisis in Syria.
Al-Ayoubi, the Syrian diplomat in London, is the fourth high ranking envoy to defect. He was preceded by the charge d'affaires in Cyprus, her husband, a diplomat in the UAE, and by the ambassador to Iraq.
A Foreign Office spokesman said al-Ayoubi was staying in a safe location in the United Kingdom and was in contact with British officials. His departure leaves five staff at the embassy and there has been no indication that they would be leaving their posts as well.
Turkey also reported that the deputy head of security for Syria's Latakia region, a regime stronghold, had defected as well.
The brigadier general was among a group of 12 Syrian officers who crossed into Turkey late Sunday, the official said, speaking on condition of anonymity because he was not authorized to speak to reporters. His defection raised to 28 the number of generals who have left for Turkey since the start of the 17-month-old uprising.
Syria's army remains mostly intact, however, and still vastly outguns the rebel forces, who are armed for the most part with assault rifles and machine guns and don't have the heavy weapons necessary to effectively oppose tanks and helicopter gunships.