Israel has blamed rocket attacks by militants for the blockade
The UK-based aid agency Oxfam has warned of catastrophe for Gaza and nearby areas of Israel if a truce agreed last June is not maintained.
Oxfam called on world leaders to do everything they could to break Israel's blockade of Gaza and urged Israel to resume supplies without delay.
Israel has shut border crossings in response to rocket attacks from Palestinian militants in Gaza.
Earlier Israel fired missiles at targets in northern Gaza.
Two Palestinian militants were injured in the attacks, while one Israeli was injured in militant attacks on the town of Sderot.
Palestinian rockets also hit near the Israeli town of Ashkelon, 15km (nine miles) from Gaza. No injuries were reported.
Oxfam said both sides would suffer if fighting continued.
"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," the agency's executive director, Jeremy Hobbs, said in a statement.
He said Gazans had been routinely denied unhindered access to fuel, medicines and essential goods for the last year-and-a-half.
"Failure of the international community to act decisively will only exacerbate human suffering and could further endanger chances for peace," Mr Hobbs added.
On Thursday, UN officials said aid for 750,000 Gazans would have to be suspended until Saturday at the earliest.
Israel earlier denied entry to a convoy carrying humanitarian supplies. It has prevented the transfer of all goods into Gaza for nearly a week, blaming continuing rocket attacks by Palestinian militants.
The current round of clashes and rocket fire began on 5 November when Israeli troops entered Gaza to destroy what Israel said was a tunnel dug by militants to abduct its troops.
Hamas responded with a barrage of rockets fired into Israel. There has been intermittent rocket fire since.
A truce between the two sides declared on 19 June had largely held. Both sides have accused the other of violating the truce, but maintain that they remain committed to it.