A police officer who opened fire on a van full of migrants and killed a two-year-old girl expressed his regrets Monday at the start of a trial that has put Belgium's immigration policy in the dock.

"If I'd known there was a child, I never would have pulled out my gun," pleaded Victor-Manuel Jacinto Goncalves, 48, on the first of two days of hearings at the court in Mons into the tragedy of May 2018.

Goncalves, a former computer engineer who joined the police in 2008, insists he fired the shot to give the van a "slow puncture" and end a high-speed motorway chase he felt put him and a fellow officer in danger.

Instead, his bullet hit Mawda, a two-year-old Iraqi Kurd who was in the vehicle with her parents and two dozen fellow migrants.

Now the officer faces an involuntary homicide charge and up to five years in jail if convicted.

For migrants' rights activists and supporters of Mawda's family, the case has come to symbolise the dangers of taking a criminal law approach to irregular migration and of "dehumanising" refugees arriving in Europe.

Goncalves, however, feels that his own humanity has been called into question.

"Before being a policeman, I'm a human being, I'm a dad," he told the court, his voice wavering. "Mawda's death left me shattered," he said in his first cross-examination.

Overnight on May 16, 2018, a van driven by suspected people traffickers -- two of them, also Iraqi Kurds, on trial alongside the officer -- left Grande-Sythe in northern France and began travelling across Belgium.

French police had the alleged gang under surveillance and had fitted the van with a GPS tracker, but their Belgian colleagues did not know this. When the van accelerated to escape them, they gave chase.

Mawda's parents left Iraq in 2015 intending to head to Britain
Mawda's parents left Iraq in 2015 intending to head to Britain BELGA / BENOIT DOPPAGNE

Goncalves drew his gun and fired towards a tyre -- only to miss and hit the bodywork. Experts told the court that the van was travelling at around 100 kilometres per hour (60 miles per hour).

Mawda, sat behind the driver, was hit in the face. She died soon afterwards in an ambulance.

Goncalves admitted that in police training it is "strongly discouraged " to fire on a moving vehicle, but he insisted that he had had no choice: "There was a danger to my life, that of my colleague and to other road users."

Mawda's parents, who left Iraq in 2015 and intended to head to Britain, ended up settling in Belgium after her death. They were granted residency in Belgium on humanitarian grounds.

The parents were represented at the two-day hearing by three lawyers and with the backing of a citizen activist group that has mobilised international celebrity support.

Before the hearing, activists hung a child's clothing outside the court and a few shouted "Justice for Mawda". Lawyers representing the family said the defendant should have faced a tougher charge than that of involuntary homicide.

"To take out his weapon, load it and fire it towards a van full of migrants -- it's not just a lack of due care, we should not minimise it like that," Selma Benkhelifa said.

The trial will continue on Tuesday, after which the court will retire to consider its verdict.