Turkish troops and allied Syrian rebels expelled the extremist Islamic State in Iraq and Syria group from the last strip of territory it controlled along the Syrian-Turkish border on Sunday, effectively sealing the extremists' self-styled caliphate off from the outside world, Turkey's state-run news agency reported.

Also on Sunday, Syrian pro-government forces backed by airstrikes launched a wide offensive in the northern city of Aleppo, capturing areas they lost last month and besieging rebel-held neighbourhoods, state media and opposition activists said.

Turkey-backed Free Syrian Army rebels have cleared the area between the northern Syrian border towns of Azaz and Jarablus, the Anadolu news agency reported. It said the advance "has removed terror organization Daesh's physical contact with the Turkish border in northern Syria." Daesh is an Arabic acronym for ISIS.

The FSA's advance shut down key supply lines used by ISIS to bring in foreign fighters, weapons and ammunition.

A Turkish army tank is stationed near the Syrian border in Suruc, Turkey, on Sept. 3. Turkish tanks crossed into Syria on Saturday to support Syrian rebels against ISIS, according to the Anadolu news agency. (Associated Press)

Meanwhile, the recapture and return to siege of rebel-held parts of Aleppo dealt a major blow to insurgent groups. They have lost scores of fighters in recent weeks in the battle to open a corridor into the city and lift the government's blockade.

After the government laid siege on Aleppo for the first time in July, the United Nations said that nearly 300,000 residents were trapped in rebel-held neighbourhoods, making it the largest besieged area in war-torn Syria. The city has been contested since the summer of 2012.

Sunday's push follows a month after insurgents captured several military academies south of Aleppo and opened a corridor into opposition-held parts of Syria's largest city and onetime commercial centre. Since then, government forces and their allies have been trying to recapture the area.

State TV quoted an unnamed military official as saying that troops are now in full control of the military academies south of Aleppo and are "chasing the remnant of terrorists." It added that all roads linking rebel-held eastern Aleppo with opposition areas outside the city "have been cut."

The Britain-based Syrian Observatory for Human Rights confirmed that government troops captured first the Armament Academy and later the Artillery Academy, laying siege on the rebel-held eastern neighbourhoods.

"The (rebel-held) neighbourhoods are under siege again," said the Observatory's chief, Rami Abdurrahman, by telephone. "The whole areas are under complete siege."

Turkey has launched two incursions into Syria since Aug. 24 in an operation designed to drive ISIS away from the border and prevent the advance of U.S.-backed Kurdish forces, which are also battling the extremist group.

​Turkish Prime Minister Binali Yildirim on Sunday defended his country's intervention in neighbouring Syria, pointing to their long shared border.

"We are there to protect our borders, ensure the safety of our citizens' lives and property, and to protect the territorial integrity of Syria," Yildirim said during a visit to the southeastern city of Diyarbakir.

"We will never allow an artificial state in Syria's north," he said, apparently referring to U.S.-backed Kurdish forces, who have taken a large swath of territory from ISIS along the border. Turkey views the Kurds as a threat and the Turkey-backed forces have clashed with them outside Jarablus.

In a press statement late Saturday, Turkey's military said the FSA took control of 10 villages from ISIS, adding that the Turkish army struck 20 ISIS targets. Since the Turkish operation began on Aug. 24, the army says it has hit 300 targets with 1,306 rounds.