Israel's latest tightening of its blockade has made an already dire humanitarian situation markedly worse," said Philip Luther, deputy director of the London-based body's Middle East and north Africa program.

"This is nothing short of collective punishment on Gaza's civilian population and it must stop immediately," he said, adding it was "nothing short of a disaster."

Meanwhile, U.N. Secretary General Ban Ki-moon also called on Israel to swiftly allow food aid and fuel supply lines to be opened for Gaza.

"The secretary-general is deeply concerned at the deterioration of the humanitarian and security situation in Gaza and southern Israel, and at the potential for further suffering and violence. He calls on all parties to uphold international humanitarian and human rights law," a statement from Ban said.

Ban "is concerned that food and other life saving assistance is being denied to hundreds of thousands of people, and emphasizes that measures which increase the hardship and suffering of the civilian population of the Gaza Strip as a whole are unacceptable and should cease immediately," it stressed.

The European Union also criticized Israel and said it deplored "the decision by the Israeli government to close the Gaza crossing points," adding "this disproportionate decision leads once again to the collective punishment of the whole civilian population in Gaza where the humanitarian situation is of great concern."

Total lockdown

U.N. food distribution to half of Gaza's 1.5 million population ground to a halt on Friday as Israel maintained a total lockdown there amid continuing violence.

Israel also cut off EU-funded fuel supplies to Gaza's sole power plant on Thursday, prompting it to shut down for want of diesel.

EU External Relations Commissioner Benita Ferrero-Waldner said in a statement that "international law requires the provision of access to essential services such as electricity and clean water to the civilian population."

"I am profoundly concerned about the consequences for the Gazan population of the complete closure of all Gaza crossings for deliveries of fuel and basic humanitarian assistance," she added.

The EU presidency also complained that some European representatives, including the French consul general in Jerusalem, who were seeking to evaluate the humanitarian and economic conditions on the ground were denied access into Gaza on Thursday.

British-based aid agency Oxfam joined the U.N., EU and Amnesty in appealing to Israel and warned that Gaza faces a humanitarian catastrophe unless Israel's blockade is lifted.

"World leaders must step up and exercise all their political might to break the blockade of Gaza," Oxfam's executive director Jeremy Hobbs said.

"As a matter of humanitarian imperative, Israeli leaders must resume supplies into Gaza without further delay…If Israelis and Palestinians alike don't exert every effort now to maintain the truce which has held since last June, the result could be catastrophic for civilians both in Gaza and in nearby Israeli towns," he said.