Syrian army reinforcements were filing into Aleppo from the city of Homs, 40 miles to the north, as rebel forces established checkpoints and sniper positions to confront a stronger military force in the battle for control of the country's largest city and commercial hub.

The Syrian Observatory for Human Rights, which has become a primary source of news because of the government's ban on foreign journalists, said Thursday helicopter gunships were hovering over two districts in the city where fighting has been taking place. It also reported that a district in the capital has been surrounded by troops and helicopters are being used in the attack. Clashes were witnessed in the city of Bab, 27 miles northeast of Aleppo.

The group claims at least 160 were killed in fighting on Wednesday in the country, 83 of them unarmed civilians. At least three rebels died Thursday, including a defected army officer who succumbed to injuries sustained leading rebels in fighting in Jarabulus, across the border from Turkey.

The BBC reported thousands of civilians were fleeing the city in anticipation of a worsening situation. Rebel forces are not likely to be able to hold their positions, according to activists on the ground interviewed by the British news agency.

An imam in Injara village near Aleppo showed reporters buildings that had been destroyed by soldiers from a nearby military base. The town has been without water or electricity in over a month and families have fled to Turkey to joins tens of thousands of others filling refugee camps in bordering countries.

The rebels, made up of a mish-mash of defected army officers and angry civilians that have taken up arms to oust President Bashar al Assad's control of the country, will likely flee their positions as they did last week when they took control of large areas of Damascus before being routed by superior forces.

The country's largest Palestinian refugee community, Yarmouk district in Damascus, also faced attacks from the army including helicopter gunship shelling. The 55-year-old unofficial refugee camp - once located outside of the city but now nested inside the urban sprawl - is home to over 150,000 Palestinians, most of who were born there.

The United Nations expressed concern Wednesday over reports the Syrian government has deployed fighter jets over Aleppo. The Syrian Air Force uses Russian made MiG-23s, which can be armed with an array of missiles that could be used against ground forces to devastating effect in an urban center.

If true, the move would mark yet "a further dangerous escalation and underlines that there are no boundaries that the Assad regime will not cross," said British Ambassador Mark Lyall Grant to the U.N. Security Council on Wednesday.

In March, Syrian rebels posted a video on YouTube showing them destroying a MiG-23MS at Abu Dhahur air base in northern Idlib province in with a Russian anti-tank missile system.