Relatives of victims of the Beslan massacre on Sunday said they were still waiting for answers, 15 years after the tragedy that left over 330 dead including 186 children.

Sunday marks the anniversary of the school siege in the town of Beslan in the Russian Caucasus when Chechen militants stormed the school, herding over 1,100 people into a gymnasium and rigging the building with explosives.

Security forces stormed the building three days later following fruitless negotiations resulting in mass casualties.

The European Court of Human Rights ruled in 2017 that Russia's handling of the siege had "serious failings" in terms of its failure to prevent the attack and its use of excessive lethal force and called on Moscow to take measures to establish the truth.

"Unfortunately, not a single point from the decision of the European court is being followed, and this is very sad," Susanna Dudiyeva, who heads the Mothers of Beslan group, told a round table about the tragedy on Sunday.

"We return again and again to September 1-3, 2004, because there are still blanks, there are still questions... we are worried that not everything has been done, because there is no objective investigation."

"We are afraid that this can repeat itself," she said.

"We will once again appeal to the investigative agencies for a new launch of the probe into the Beslan attack," she said.

A ceremony and religious service was held to mark the anniversary in the school gymnasium, now converted into a memorial, packed with grieving relatives, and adorned with stuffed toys, candles and red carnations.

'Above the law'

The ECHR decision which looked at evidence and testimonies collected by an independent investigation said Russian authorities had information about the planned attack but failed not only to prevent it but to boost security or warn the public.

It said the security operation lacked leadership and eventually used heavy weaponry like flame throwers on a building full of hostages.

It suggested measures aimed at "drawing lessons from the past" and "deterring similar violations in the future."

Russia's official investigation claimed that the victims died when two bombs planted in the gym went off, which forced the storming of the building by special forces.

However the independent investigation said most of the explosive devices in the gym remained intact, while many bodies of the victims were found elsewhere in the school, as if they were used as human shields by regrouping militants.

Post-mortems to establish cause of death were not carried out, and the official version "had not one word of truth," according to a new documentary released last week by Novaya Gazeta independent newspaper.

"The main goal was to eliminate terrorists, not to rescue people," the film says, accusing authorities of giving the public false information during the hostage crisis and refusing to negotiate via Chechen separatist leader Aslan Maskhadov.

An open letter published Sunday by Ella Kesayeva, who lost several relatives in the siege and has been an outspoken critic of the official probe, called on five Russian generals involved in the Beslan operation to answer the many allegations.