Former Irish President Mary Robinson says Israel is using "disproportionate" force in Gaza.
More than 500 Palestinians have been killed so far in the two-week campaign.
The United Nations Security Council is demanding an immediate end to the fighting, but doubts have been cast on its ability to secure a ceasefire.
Ms Robinson says the UN has proven itself to be "ineffective", but says Israel's offensive must be brought to a swift end.
She said: "It (The UN) is not effective in our world today, in the way that it should be. Now in fairness, Ban Ki Moon is at the moment in Qatar and the security council yesterday passed a resolution about the situation in Gaza.
"The disproportionate use of violence has to stop, there's no doubt about that, and I hope it will stop today."