"International humanitarian law prohibits starvation of civilians as a method of warfare and ... it is also prohibited to impose collective punishment on civilians," Pillay said, according to Reuters.
"I have consistently reported to member states that the blockade is illegal and must be lifted."
Pillay reportedly added that should the siege be proven legal under international law, Israel's assault against six boats en route to Gaza which left nine activists dead should be analyzed within the framework of Israel's obligation to allow the entry of humanitarian aid into Gaza.
On Wednesday, the UN Human Rights Council voted to dispatch an independent, international probe into violations of international law result from Monday's attack, which occurred in international waters.