Gaza could become completely dependant on aid within weeks because of economic damage caused by Israeli restrictions on border crossings, the UN has said.
Karni is the main transit point for goods between Israel and Gaza
All 600 garment factories in Gaza have closed, because they cannot import raw materials, a UN relief agency said.
And more than 90% of factories involved in the construction industry have shut down and laid off workers.
A senior official from the UN Relief and Works Agency called for the opening of the main crossing point into Gaza.
Unless restrictions are eased, the agency (Unrwa) said, Gaza may become "virtually 100% aid dependent".
Filippo Grandi, the agency's deputy commissioner general, said in a statement: "I appeal to the Palestinian authorities, to Israel and all other parties to take immediate steps to open up the Karni Crossing, to imports and exports, as well as humanitarian goods...
"Only this will allow the little that remains of Gaza's economy to survive."
He added: "Gaza risks becoming a virtually 100% aid-dependent, closed down and isolated community within a matter of months, or even weeks, if the present regime of closures continues."
Israel has sealed off Gaza from the outside world since the takeover of Gaza by the radical Hamas group.
Hamas' charter calls for the destruction of the Jewish state and Israel says the restrictions are necessary for security reasons.
The Rafah border crossing, in the south of Gaza, is the only way in or out of the Palestinian territory that does not pass through Israel - although when it is open the Israeli authorities monitor traffic electronically.
The BBC's Matthew Price, in Gaza, says Gaza is sealed off, politically and economically.
It is now almost entirely dependent on aid, with practically everyone reliant on handouts provided by the United Nations, our correspondent says.
The Palestinian Shippers' Council estimates the total losses of the private sector since the closure of the Gaza border in mid-June are $35m (£17.3m), according to Unrwa's latest report on the humanitarian situation there.
Since 19 June, the average numbers of trucks carrying goods allowed through Gaza's crossings each day has fallen from 238 to 118 - with food supplies representing 89% of commercial goods transported into Gaza.
About 50% of the Gaza Power Plant's production capacity has been lost due to delays in maintenance of its turbines, Unrwa says.