BEIRUT — Syrian troops advanced in and around the northern city of 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 opposition fighters in Syria's largest city since they entered it two years ago. Aleppo, once Syria's commercial center, has been carved up into rebel- and government-controlled areas since an opposition offensive in mid-2012.

The push also comes a month after extremist fighters of the Islamic State group seized territories straddling Syria and neighboring Iraq 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 Aleppo.

Abdurrahman and an activist based near the city who goes by the name of Abu al-Hassan said Monday's fighting was concentrated near an army base known as the Infantry Academy 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.

The move against the rebel-held areas comes two months after hundreds of Syrian opposition fighters withdrew from parts of the central city of Homs that they had held for nearly two years despite a government blockade. Homs, once the center of the revolt against President Bashar Assad, is currently under government control.

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

A Syrian army officer told state TV that troops now control a main highway north of Aleppo and "we have closed a belt of up to 80 percent from the north." A correspondent for the channel said that military experts were dismantling bombs placed by rebels in the industrial area, adding that large amounts of weapons were captured.

Government forces control much of western Aleppo and have closed down most of the city's southern entrances, and are currently attempting to close the northeastern gates.

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

State-run news agency SANA said the army continued its crackdown on "terrorist'" 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 protests against President Bashar Assad's rule. It turned into an armed uprising after some opposition supporters picked up weapons 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.