Fighting is heating up in Aleppo, Syria's largest city with three million people. The United Nations says 200,000 Syrians have left over the past 10 days as the government trains its mortars, tank and helicopter gunships on the neighbourhoods seized by the rebels.

"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 UN 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 UN 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.

Aleppo is some 50 kilometres 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 al-Assad's rule.

Turkey is deploying more troops to the border, sending tanks, armoured 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.

UN observers come under attack

UN Secretary General Ban Ki-moon said Monday that Syrian forces had attacked a convoy carrying the head of the UN observer mission. He again told the government to halt its violent crackdown on those fighting to overthrow Assad's regime.

"Yesterday, the convoy of Lt.-Gen. Babacar Gaye was [attacked]," Ban told reporters.

"Fortunately, there were no injuries," Ban said.

UN spokesman Farhan Haq added later that the UN had learned that the five-vehicle convoy was hit by small arms fire Sunday in Talbisa,17 kilometres away from Homs. Two of the five vehicles were hit by bullets.

The UN observer mission's latest 30-day mandate is due to expire in August. The mission has largely been sidelined by the violence.

"It's important, again, that the Syrian government must fully co-operate with the UN mission and must cease these violent measures. We are deeply concerned that they are using all kinds of heavy equipment, including military airplanes, attack helicopters and heavy weaponry," Ban said. "This is an unacceptable situation. Every day, more than 100 people are being killed."

Aleppo residents on the run

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, which were posted to the internet Monday, 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 neighbourhood 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 neighbourhood has been without electricity since the morning.

Lt.-Gen. Gaye, the head of the UN observer mission in Syria, expressed concern over the 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 ceasefire that has never been observed by either side.

U.S. Defence Secretary Leon Panetta said late Sunday that the use of heavy weapons, particularly helicopters, is just another nail in Assad's coffin. He spoke during a stopover in Tunisia as he kicked off a Mideast tour expected to focus on the unfolding crisis in Syria.

Diplomatic defections

Syria's top diplomat in London said he could no longer represent the regime and defected Monday.

Britain's Foreign Office said that Khaled al-Ayoubi, the chargé d'affaires, told officials that he wasn't willing to represent the regime any longer, the latest high-profile defection of a diplomat from Syria over the bloody crackdown on the opposition since March 2011.

Al-Ayoubi, the Syrian diplomat in London, is the fourth high ranking envoy to defect. He was preceded by the chargé 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.