Kofi Annan, the UN secretary general, said he was "extremely concerned about the dangerous situation in the occupied Palestinian Territory".
"The passage of foodstuffs and other essential supplies through the Karni commercial crossing should be ensured and restrictions on movement and access for UN agencies should be lifted forthwith," Annan said in a statement released by his office on Saturday.
Israel has carried out air strikes on Gaza's infrastructure. The only power station was destroyed effecting hospitals, flour mills, water and sanitation systems.
Annan urged Israel to replace damaged Palestinian equipment.
UN humanitarian groups warned that Gaza is on the brink of disaster unless "immediate and urgent" action is taken.
In a separate statement, the UN Humanitarian Agencies working in the occupied Palestinian territory said they were "alarmed by developments on the ground, which have seen innocent civilians, including children, killed, brought increased misery to hundreds of thousands of people and which will wreak far-reaching harm on Palestinian society".
"An already alarming situation in Gaza, with poverty rates at nearly 80 per cent and unemployment at nearly 40 per cent, is likely to deteriorate rapidly, unless immediate and urgent action is taken"
UN Humanitarian Agencies' statement
"An already alarming situation in Gaza, with poverty rates at nearly 80 per cent and unemployment at nearly 40 per cent, is likely to deteriorate rapidly, unless immediate and urgent action is taken," the statement said.
The three-page statement drawn up by six UN agencies said Gaza is "on the brink of a public health disaster", which includes a shortage of medicines. Also, it warned that children in the territory "are living in an environment of extraordinary violence, insecurity and fear".
The statement demanded that Israel repair the power station it destroyed.
Israel has rejected the charges and denied that there is a developing humanitarian crisis.
Yariv Ovadia, Israeli foreign ministry spokesman, said the operation "has been tailored to avoid civilian casualties while bringing sufficient pressure to bear on the Hamas-led government of the Palestinian Authority" to order release of a captured soldier and halt the rocket attacks.
He said Israel is allowing transfer of fuel and food into Gaza through crossing points from Israel "despite continued attempts by terrorist groups to attack these facilities".