The town was empty of its inhabitants, and the marks of fighting and bombing were everywhere, said an AFP journalist who visited the town on a government-organised trip.
An army commander on the ground said troops entered Adra at five different locations, "and fought the armed men who were trying to cover for their retreating friends. Several (rebels) were killed."
The AFP journalist said Adra was completely empty of its residents, and that the walls of the buildings at the entrances to the town were riddled with the marks of shelling and gunfire.
Broken glass and shrapnel filled the streets of the town, to which the power supply had been cut off.
The rebels captured Adra in December last year and regime forces have been fighting to retake it ever since.
They have advanced incrementally in the town, one of the country's biggest industrial zones strategically located northeast of Damascus.
On Thursday, the AFP journalist saw tunnels the rebels had dug under the buildings to move from area to area of the town, as well as holes they had blasted through walls separating apartment buildings.
Barriers could also be seen in the town, presumably used by the fighters to protect themselves from the loyalists' view.
On Adra's walls, the rebels had painted slogans.
Referring to Syria's biggest rebel alliance, one of them read: "The Islamic Front is fighting here, and it will return to Damascus."
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Another said: "We the fighters, we the commanders... we love martyrdom."
The security source said the army had taken control on Thursday of an area in the town used to house workers, after securing the highway and the industrial zone.
He acknowledged, however, that Adra's small Old City remained under rebel control.
The Syrian Observatory for Human Rights also confirmed that the army now had full control over the area used to house workers.
The field commander said a large number of rebels had withdrawn to the Old City as the army advanced.
The AFP journalist in Adra could hear the sound of gunfire and shelling from the direction of the rebels' outpost.
Branding the rebels' retreat as "a real defeat" to the armed opposition, the security source said: "The rebels withdrew after they felt threatened by the army's advance."
Some 150,000 people lived in the area of Adra that houses workers before Syria's war broke out in March 2011.
The army's latest advance will allow the government to further tighten its seige of the rebel-held Eastern Ghouta area, east of Damascus, according to the Observatory.
After a series of stunning rebel advances around Damascus in 2012, the army has slowly reclaimed a string of hotspots in the past two years.