Steve: I have no wish to side with Hamas, but I can't find any reports about Hamas withholding aid.

"Goren also said that in coordination with the UN, the army was opening a new humanitarian zone in Khan Younis, a city in the south of the strip, which he said would not be targeted by Israeli bombing.
When asked by the Guardian for details, the colonel did not give the location or size of the new “safe zone”, but said it could hold “hundreds of thousands of people” in a “relatively open area”.
Goren also accused Hamas of using the area’s civilian population to further its operational goals, withholding stockpiles of water, food, medical supplies and petrol in order to fuel the humanitarian crisis and slow the Israeli advance. Cogat, citing coordination with the UN, said there was “not a food and water shortage” in the strip, although the supply was “not at a normal level” and access was more difficult in some areas than others."

Source: The Guardian admittedly you have to read almost to the end to find it, while the Telegraph includes it as a headline (but that article is behind a paywall)