The Israeli authorities have delayed re-opening the main goods crossing into Gaza, despite warnings of shortages of essential supplies for Palestinians.
Palestinian farmers have been protesting at the lengthy closure
On Wednesday, the Israeli army had issued a statement saying the crossing would open again on Thursday.
Israel has kept the crossing closed for nine straight days due to security concerns, officials say.
Palestinians describe the closure as collective punishment, while the UN says it has caused severe shortages.
"This is getting to precarious levels," said David Shearer, head of the UN Office for the Co-ordination of Humanitarian Affairs, in an interview with Reuters news agency.
Stocks of wheat, sugar and cooking oil could begin to run out within days, the UN said on Wednesday.
"We had already informed the merchants that Karni would open," said the Palestinian official in charge of the border, Salim Abu Safiyeh.
"The continued closure is causing humanitarian and economic harm," he said.
Palestinian farmers had been planning to dump hundreds of tonnes of spoiled produce that they would normally have exported through Karni crossing.
Palestinian economic officials say Gaza's agriculture sector - including market gardens which once earned Jewish settlers millions of dollars in exports - is on the verge of collapse because of the closures.
The Karni crossing has been shut for 35 days during 2006, causing losses to farmers and exporters of 57m euros ($68m), the Palestine Economic Development Company says.
A US-brokered deal last November specified that the crossing would be kept open unless there was an immediate security threat.
On Tuesday, the Jerusalem Post reported that the Palestinians had rejected an Israeli offer to use the smaller Kerem Shalom crossing in southern Gaza - while Karni remained closed - on the grounds that it may turn into a permament arrangement.
Israel withdrew its settlers and the troops who protected them in 2005 after a 38-year occupation. It retains exclusive control of Gaza's airspace and territorial waters and the border with Israel.
Palestinian militants frequently fire home-made rockets toward Israel from Gaza, causing fear and disruption but few casualties.
Israel's acting Prime Minister Ehud Olmert has vowed to use an "iron fist" against Palestinian militants amid fears of a rise in violence.
The Israelis are holding a general election later this month, which places pressure on the government to clamp down on anti-Israeli attacks.
There has been a spate of shootings and stabbings in the occupied West Bank in recent days, leaving one Israeli civilian dead and five wounded.
"I have issued an order to all the security forces of Israel to use special means to confront the buds of terrorist action," Mr Olmert told a news conference.
In recent weeks, the Israeli army has launched arrest raids in the West Bank - in which a number of Palestinians have also died - and killed militants in Gaza with air strikes.
But it denied responsibility for the car bombing in Gaza City on Wednesday that killed Islamic Jihad's top military commander in Gaza, Khaled Dahdouh.
Mr Olmert also reiterated his refusal to meet the Palestinian PM-designate, Ismail Haniya, of the Hamas militant group - which is dedicated to Israel's destruction and which won Palestinian parliamentary elections in January.
Separately, the leader of the opposition Labour Party, Amir Peretz, has met Palestinian Authority President Mahmoud Abbas - who has also been shunned by Mr Olmert - in Jericho.
Mr Peretz told journalists it was important to strengthen ties with Mr Abbas in order to weaken extremists.