Portraits show Syrian athlete Mireille Hindoyan, 20, (L) and her 12-year-old brother Arman, who were killed during shelling on the government controlled area of Aleppo on September 30, 2016 (AFP Photo/GEORGES OURFALIAN)

Aleppo (Syria) (AFP) - On the day she was killed, Syrian star swimmer Mireille Hindoyan had decided to skip her daily swim at an Aleppo pool to work a shift at her family's supermarket near the frontline.

Not long after she arrived, a rocket fired by rebels just across the frontline in the war-ravaged city slammed into the shop, killing the 20-year-old and her 12-year-old brother Arman.

At the family home, her mother Betty sat pale and drawn, dressed in mourning black and weeping as she looked at the makeshift memorial to the two of her children killed in the September 30 attack.

"They went to God, maybe living with him is better than living in war," she said in tears, looking at a spread of Mireille's swimming medals and photos of the dark-haired girl and her brother.

Her middle child, 19-year-old Movses, survived the attack but was badly wounded.

An ambitious athlete and third-year university student, Mireille was a national short-distance swimming champion who dreamt of becoming a nutritionist.

Her family, Armenian Syrians, live and work in a neighbourhood along the frontline that has divided Aleppo city between rebel control in the east and government control in the west since 2012.

"Mireille usually goes to the pool every day around 11 am to swim, but on that day she came to our supermarket and told me she didn't feel like swimming," said her mother, 42.

"I asked her to go home, but she refused," she added, her voice breaking.

- Calling for her children -

Minutes after she tried to convince Mireille to leave, a huge blast shook the store, knocking out the lights and throwing Betty and her three children to the ground.

Her son Movses screamed: "Mum, my hand is gone."

Betty called out to Mireille and Arman "to stay close to me, thinking they were alive but just lying on the ground".

"As the dust began to settle, I looked around me and I saw Arman, my youngest son, on the floor. His intestines were lying outside his body. He was dead," she said in a trembling voice.

Betty then called out for Mireille.

"I found her on the ground too, she was missing both her hands and legs. She was bleeding. I couldn't lift her because I was wounded and bleeding too, and I wasn't able to move."

Hundreds of people have been killed in western Aleppo city by rebel fire since the conflict began, according to the Syrian Observatory for Human Rights monitor, which has also recorded thousands of deaths in the rebel-held east in government air strikes, barrel bomb attacks and artillery fire.

In recent weeks, the government has stepped up its assault on the east in a bid to recapture it, and rebels have increased the pace of their crude rocket fire on western Aleppo.

Rights groups have condemned both sides for indiscriminate fire that kills civilians.

Syrian state news agency SANA said 13 people were killed in rebel fire on western Aleppo on the day Mireille and Arman died.

- 'Mireille gave us strength' -

More than 300,000 people have been killed and over half Syria's population displaced by the conflict since it began with anti-government demonstrations in March 2011.

Among those displaced are millions who have become refugees, some braving leaky boats to cross the Mediterranean like fellow Syrian swimmer Yusra Mardini.

She gained international attention after surviving near-drowning and reaching Germany, and competed in this year's Rio Olympics as part of a refugee team.

Mireille had won medals in both Syria and Armenia and was coaching young female swimmers in Aleppo as well as training to swim in open-water long-distance competitions.

"She was very ambitious... She remained an athlete even after the war began," said coach Wanis Salahiyan, who had trained Mireille since she was five.

"Mireille was very ambitious and had big dreams... She always wanted to be the best at whatever she was doing, especially in sports," her mother added.

Mireille's father Viken, 56, tried to remain composed in front of his wife as they looked at pictures of her and Arman.

"We've lost material things before, shops and warehouses. But this time our loss was huge, I've lost my son and daughter," he said.

Both her parents described Mireille's unusual fortitude, depicting her as a rock for the family.

"It was Mireille who gave us strength," Viken said.