After Mohammed Walo's Aleppo neighbourhood was recaptured by Syria's army, he crossed into the well-stocked government-held west for groceries. On the way back, he was stopped at a checkpoint and conscripted.

"I was passing through an army checkpoint, and on my way back, they told me I was wanted for the army reserves," Walo told AFP.

It was the first time the 35-year-old technician had left his Haluk neighbourhood since rebel groups overran it four years ago.

"I have to come back to serve in the army for my son who is very dear to my heart, so that he can inherit a country that isn't in ruins," Walo said, with tears in his pale green eyes.

He is one of hundreds of men who have been swept up to perform their compulsory service or enter the army reserves since the government began recapturing territory in east Aleppo from rebels.

After a month-long assault, Syria's army and allied militias are closing in on the shrinking enclave still held by rebels, with tens of thousands of residents streaming into government-controlled zones.

The flows have boosted Syria's army reserves, according to General Habib Safi, who runs the military police station where Walo and more than 200 other new conscripts are being debriefed.

In Syria, males above 18 years of age are required to complete up to two years of military service, after which they are automatically enlisted in the army reserves.

Exemptions are issued for students, males who are the only child, sole breadwinners for families, and anyone with handicaps.

The army's 300,000-strong pre-war force has been halved by deaths, defections and draft-dodging.

President Bashar al-Assad, in an interview Wednesday, said it was "obvious the Syrian army is not to be as strong as it was before. But what we have is determination to defend our country. This is the most important thing."

- 'Allegiance to homeland' -

Gesturing to the new conscripts in west Aleppo's Feid district, Safi said: "The displacement is providing the army with groups of men. This is just part of those that are here."

Around 700 men had been recruited into the armed forces since the escalation in Aleppo, "and the number is growing", the general said.

As families exit east Aleppo into regime-controlled territory, men undergo background checks to identify anyone wanted for draft-dodging, desertion or any other violations.

Any person eligible then spends a week at government "collection centres" while the recruitment process is finalised.

"We monitor his inclinations and his competencies, as well as his allegiance to his homeland. When this is genuine, he is immediately admitted into the ranks of the armed forces."

The United Nations has raised concerns that hundreds of Syrian men may have gone missing after heading from east Aleppo into government-controlled zones.

The UN's Independent International Commission of Inquiry on Syria said it had reports of "enforced disappearances and forced conscription", with men of military age appearing to be most at risk.

New recruits who spoke to AFP near Syrian security officials said they were being treated well, including Mohammed Ali, 19, who told AFP he fled eastern Aleppo last Saturday.

"I knew that I still had to do my military service, but I was too scared. After witnessing the clashes, I decided to hand myself in," the young man said.

"I never carried arms (against the government). I just sold water from a cistern fixed on top of my car."

- Families crowd outside -

Ahmad al-Qassem, 23, told AFP he was forced to join a rebel group in Aleppo under threat of torture, and that he was taught to fear Syrian regime forces for many years.

"They told me, 'They will kill you.' But I saw on television that a lot of people were getting their status regularised," he said.

He and other men from his neighbourhood handed themselves in to the army earlier this week, and he hasn't seen his family since.

Outside the police station, dozens of people -- mostly women and children -- waited impatiently to see their recently-recruited relatives inside.

"I'm waiting to see my son Ahmad, who's inside the centre," said Iftikhar, 45, who left the Tariq al-Bab district of east Aleppo two weeks ago.

"We didn't know that he had to go into the reserves," she said, pulling her black headscarf tighter around her in the December cold.

"God protect him."

Amin Derzi, 50, came to the station to check on his son, who was detained when the family left the Salhin district several days ago.

Derzi's 27-year-old son was wounded two years ago in a rocket attack that left him without fingers on one hand.

"We got him treated, but it wasn't enough... I haven't seen him since yesterday, and I want to check on him and give him his medication," he said.