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

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

"The latest attack does not mean that Aleppo will fall. It is going to be a very difficult battle,'' he said.

Abdurrahman's Syrian Observatory has a network of activists across Syria. Al Jazeera cannot independently verify the information because of restrictions on reporting from inside the country.

Abu al-Hassan, an activist based in Aleppo, told Associated Press news agency that fighting concentrated near an army infantry base that rebels captured two years ago.

If successful, the government forces' advance would be the biggest blow to the opposition since they entered the northern city two years ago.

Aleppo, once Syria's commercial centre, has been carved up into rebel- and government-controlled areas since opposition fighters launched an offensive in the country's north in mid-2012.

Monday's reported push also comes a month after fighters of the Islamic State group seized territories straddling Syria and neighbouring 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.

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

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 conflict began in March 2011 as largely peaceful protest against President Bashar al-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.

Al Jazeera