On his wedding anniversary on Wednesday, Abdulhamid Yusuf will have nothing to mark but a chemical attack that killed his wife and their two babies.

At least 80 people died on April 4 last year when war planes dropped sarin gas on his hometown of Khan Sheikhoun in north-west Syria.

The chemical attack on the rebel-held town was one of the most shocking of Syria's seven-year war, causing global outrage and rare retaliatory air strikes by the US.

"I've been deprived of part of my body, of my soul," said Yusuf, 29, breaking into tears as he sat in the garden of his empty home.

An image of him holding the lifeless bodies of his 11-month-old twins – Aya and Ahmad –spread around the world. His wife Dalal and 16 other relatives, including a brother, nephew and many cousins, were also killed.

Yusuf's grief and anger remain raw 12 months on as he visits the cemetery to weed the graves of his loved ones.

"I won't be able to start over. I won't forget the past," he said.

Khan Sheikhoun lies in Syria's Idlib province, the last in the country to remain largely beyond the control of President Bashar Al Assad.

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UN war crimes investigators said they have evidence Syrian government forces were responsible for the attack on Khan Sheikhoun. The allegations have been rejected by the Assad regime and its ally Russia.

"We want the international community to take a strong stand ... Assad needs to pay," said Yusuf.

Thirty children were among those killed in the early morning raid, the Syrian Observatory for Human Rights said.

Witnesses said people dropped to the ground, convulsing violently, some with white foam pouring out of their mouths.

Ahmad Al Yusuf, 20, lost his parents and two young brothers –Mohammed and Anwar – on a day he says he will never forget.

His mother had woken him up to perform morning prayers before he headed out to work on his grandfather's land.

After the strikes hit, he rushed back home to find his neighbour sitting on the ground, shaking uncontrollably and incapable of talking – but staring straight at him.

"I'll never forget that day or those details," said the young man with a short haircut, who now runs the family's convenience store on his own.

"I lost all my family – everything that was dearest to me."

He clings on to their memory even as he adapts to his new life alone.

"Whether I'm coming or going at home, I always see them in front of me."

The deadly strikes on Khan Sheikhoun sparked international condemnation and caused the United States to fire 59 Tomahawk cruise missiles at a Syrian airfield allegedly used in the attack.

But the bereaved residents feel nothing more substantial has been done to hold those responsible to account.

Mohamed Al Jawhara, a 24-year-old with blond hair and blue eyes, lost his parents, nephew and several cousins.

"It was such a shock. How do you bear seeing them all die in a single day?"

The Khan Sheikhoun attack was yet another low point in seven years of Syria's war, which started in 2011 with the brutal repression of anti-government protests.

Multiple rounds of UN-backed talks have failed to stem the fighting, and Russia-backed regime forces have instead made significant military gains across the country.

Mr Al Jawhara expresses frustration at what he sees as the insufficient response of the international community in holding Mr Al Assad to account.

"We hoped he would be tried and have to pay" for what he did, says the student, who aims to be a teacher one day.

World leaders "have made statement after statement, but in the end they have been weak".