The Syrian Army has begun what it says is an antiterrorism operation against armed opposition forces in Aleppo, Syria’s economic center, Syrian state news agency SANA said on Friday.

Fierce fighting broke out on Friday between pro- and anti-government forces, mostly members of the Free Syrian Army, in several districts throughout the city in what amounts to the Syrian Army’s colossal push to regain control of key territories across the country.

Opposition fighters told French media that the battle for Aleppo could prove decisive. If the rebels clinch a victory, they would control Syria’s second most important city, as well as a crucial border crossing with Turkey through which opposition-bound arms and material have poured in recent months.

The United States has expressed concern over the assault and what it believes to be the buildup of heavy weaponry around the area.

In Damascus, meanwhile, the Syrian Army is preparing to sweep through the southern districts of the city. An eyewitness there told RIA Novosti by telephone that helicopters were firing on districts neighboring the Yarmouk Palestinian refugee camp.

According to the UN, the Syrian conflict has claimed more than 16,000 lives and created about 230,000 refugees, while around 1 million people require humanitarian assistance.