Rows of dead children, many of them infants, their lifeless limbs contorted and draped over one another – this is the horrific scene captured in amateur video from the Syrian town of Houla. The massacre in Houla, near Homs, began on Friday with government shelling of civilian homes. According to local activists, it carried on into the night, when roving bands of militia went from house to house killing women and children. The UN says at least 90 people were killed in the onslaught, 32 of them children under the age of 10.

After fifteen months of unrest, the world has grown accustomed to the daily brutality in Syria, but this act of inhumanity we cannot ignore and we cannot forget.

'Horrific scenery'

More than 10,000 Syrians have died since the uprising against President Bashar al-Assad began in early 2011. Amid that sustained violence, few episodes have matched the horror that is now emerging from Houla. Reports of a massacre surfaced on Friday evening, and were soon paired with extremely graphic amateur video, showing the butchered bodies of infants – pyjamas soaked in blood, some with crushed skulls. The horrifying video, uploaded to YouTube late on Friday, can be found here. (We must warn you that it is devastating to watch.)

Residents of Houla have said the killings were carried out by the army, who shelled the town, and the regime-aligned militia known as the "shabiha", who murdered with guns and knives. The Assad government has, as always, blamed "armed terrorist groups".

Many of the dead have injuries consistent with shellfire, but Avaaz cannot confirm whether or not the children were attacked with knives. The UN, which has nearly 300 monitors on the ground in Syria, confirmed "the use of small arms, machine gun, artillery and tanks".

Witness accounts and testimonies are still trickling from the scene. According to Abu Jaafar, an activist based in Houla:

At around 6pm, shabiha militia from the neighbouring villages, expected to be backed up with security forces, stormed into approximately 60 houses in the neighbourhood. When we managed to reach the neighbourhood, we were shocked by the horrific scenery as blood and traces of bullets were visible everywhere. We found 13 dead people in the first house we entered; 12 children and a woman. The youngest child was a two-year-old girl.

What peace plan?

On Saturday, the citizens of Houla dug mass graves. And as the dead were buried, governments around the world condemned the massacre. British foreign secretary William Hague called for an emergency session of the UN security council; Laurent Fabius, France's foreign minister, suggested an immediate meeting of the "Friends of Syria" group in Paris; Hillary Clinton, the US secretary of state, labelled the killings an "atrocity" and said that Assad's "rule by murder and fear must come to an end".

One of the more troubling aspects of the Houla massacre is that hundreds of UN monitors were on the ground, as part of the internationally brokered Annan peace plan to end the unrest, but were powerless to intervene. UN secretary general Ban Ki-moon denounced the attack as a "flagrant violation of international law"; but that has not eased anger over the monitors' and the international community's ineffectual response to the Syria crisis.

A doctrine of hate

Perhaps most worryingly, the attacks appear to have been motivated by sectarian hatreds: the town of Houla is Sunni Muslim; the surrounding villages are mostly Alawite – belonging to the same unorthodox offshoot of Shia Islam as Assad and many of his top officials. Now some armed rebels and militants are threatening to abandon the Annan peace plan, and fears of sectarian reprisals – already well-documented – are growing.

Friday's massacre was more than a one-off act of inhumanity carried out by a few individuals – it was the fruit of decades of authoritarian rule, maintained through fear and sectarian suspicions. Assad's most potent weapon is communal distrust, and the evil committed in Houla has strengthened that devious tool. Now the world must stop him before he uses it to greater effect.