SAADA, Yemen (Reuters) - Hafidh Abdullah al-Khawlani watched as his father arranged some plastic flowers on top of one of dozens of small, fresh graves lining the dusty new cemetery in Saada, north Yemen.

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Together they recited some verses from the Koran, and prayed, hands cupped. Some graves are marked with little more than a few jagged rocks, but all are topped with portraits of the children buried there, killed last month when their school bus was hit by a Saudi-led coalition air strike.

Hafidh, 11, survived the air strike, but his older brother Waleed was among the 40 children who died on the bus as it drove through a market in Dahyan, a town in the province of Saada.

Mountainous Saada is the stronghold of the Houthis, the group which controls much of Yemen and the capital Sanaa and which Riyadh has been fighting since 2015. The terrain makes ground battles difficult and the region has been the target of many air strikes.

On Wednesday, thousands of Yemenis massed in Saada city, calling for the prosecution of the Saudi-led military coalition over the air strike on the bus, which drew horrified reactions from around the world.

“We were coming back (from a school trip) and my brother was sitting next to the bus door,” Hafidh said.

“On the bus, the teacher told us that everyone should hold his brother’s hand, and then mine was martyred.”

So many died that a new cemetery had to be built to accommodate the bodies. Inside the hastily erected wall, some of the graves are still empty.

After initially saying it was a legitimate strike and accusing the Houthis of using children as human shields, the coalition said on Saturday it accepted that the attack was unjustified and pledged to hold accountable anyone who contributed to the error.

The rare concession followed mounting international pressure, including from allies, to do more to limit civilian casualties in the 3-1/2 year civil war that has killed more than 10,000 people and caused a humanitarian disaster.

The conflict shows no sign of being resolved. United Nations-mediated peace talks on Yemen appeared to be in jeopardy on Thursday with no sign in Geneva of the delegation from the Houthi movement on the first day.

In Hafidh’s home, a modest house of baked mud bricks in the outskirts of Saada city, a portrait of Waleed hung on the wall, his image laid over a green background - the color of the Houthis - and printed with their slogan: “Death to America, Death to Israel”.

The boys had been on a school trip to visit the graves of Houthi fighters killed in the war when their bus was struck.

“The bus was completely destroyed. When I asked about my kids, they warned me that warplanes would strike again ... we searched and found body parts and other wounded children, but could not see mine,” said Hafidh’s father, Abdullah al-Khawlani.

“I found Hafidh in the hospital and asked him about his brother Waleed ... he said they took him and his eyes were closed.”

A few blocks away lies the mangled wreckage of their bus, now a playground for the neighborhood children.