Syrian troops advance in and around the country's largest city of Aleppo

Syrian troops advanced inside and near northern Aleppo on Monday in what appears to be an attempt to lay siege to rebel-held parts of the country's largest city, activist said Monday.

If successful, it would be the biggest blow to the opposition since they entered the northern city two years ago. Aleppo, once Syria's commercial center, has been carved up into rebel- and government-controlled areas since opposition fighters launched an offensive in the country's north in mid-2012.

The push also comes a month after extremist fighters of the Islamic State group seized territories straddling Syria and neighboring Iraq, mostly running across the Euphrates river where they have declared a self-styled caliphate. Most of the land was seized in June during a push across Iraq.

Rami Abdurrahman, who heads the Britain-based Syrian Observatory for Human Rights, said Monday that reinforcements, including members of the elite Republican Guards and allies from Lebanon's Hezbollah group, had recently arrived in the city.

Abdurrahman and an activist based near Aleppo that goes by the name of Abu al-Hassan said Monday's fighting concentrated near an army infantry base that rebels captured two years ago.

"The latest attack does not mean that Aleppo will fall. It is going to be a very difficult battle," said Abdurrahman, whose group has a network of activists around the country. He said the aim of government forces currently is to try capture Aleppo's northern district of Handarat to be able to further close in on rebels.

Syrian government forces last week seized a key industrial area, allowing them to choke off rebel-held parts of Aleppo.

Marea said government forces captured the village of Kafr al-Saghir and Moqbila, just north of Aleppo tightening the grip on the city's entrances.

State-run news agency SANA said the army continued its crackdown on "terrorists'" hideouts and had killed and wounded scores. The government refers to opposition fighters as terrorists.

Syria's crisis began in March 2011 as largely peaceful protest against President Bashar Assad's rule. It turned into an armed uprising after some opposition supporters picked up arms to fight a brutal government crackdown on dissent. It gradually became a civil war, in which more than 160,000 people have been killed, according to activists, and nearly a third of Syria's population of 23 million has been displaced. A third of those killed have been civilians.