Seven of the guards were killed in Rafah and five in Khan Yunis, according to the civil defense spokesperson Mahmud Basal.
The occupation has once again targeted those who secure the aid trucks, he says, adding that tens of people were injured in the attack.
The trucks were carrying flour and were on their way to a warehouse belonging to the UN's organization for Palestinian refugees (Unwra). According to witnesses, residents looted flour from the trucks after the attack.
The UN and aid organizations have repeatedly warned of the acute humanitarian crisis in Gaza.
According to the civil defense, 21 people, including several children, were killed when Israeli fighter jets attacked two residential buildings near Nuseirat and the city of Gaza.
15 were killed, including at least six children, and over 17 injured were found after the Israeli bombing, says Mahmud Basal about the attack in Nuseirat.
Six others are reported to have been killed in Gaza City.
On Wednesday, the UN General Assembly adopted a non-binding resolution calling for an immediate and unconditional ceasefire in Gaza. It has been rejected by the US, which has veto power when the Security Council considers binding resolutions.
