An Israeli military spokesman said the Erez border crossing, the main passage for people between Israel and Gaza, was closed after two mortar bombs fell in the area.
Israel temporarily opened three crossings into Gaza on Friday, allowing a limited amount of food and fuel to reach blockaded Gazans.
The Israelis allowed 40 lorries loaded with grain and bird feed to pass through the Karem Abu Salem crossing.
Another 40 lorries carrying food supplies are expected to reach Gaza via the Karni crossing later on Friday, with 400,000 litres of industrial fuel destined to reach Gaza's only power station via Nahal Oz.
About 120 tonnes of cooking gas is also expected to reach the Strip via Nahal Oz.
Ehud Barak, the Israeli defence minister, said he ordered the crossings opened for essential humanitarian supplies in response to numerous requests from the international community.
Ayman Mohyeldin, Al Jazeera's correspondent in Gaza, said while the move would be welcomed, the amount of supplies allowed through was nowhere near enough to genuinely ease conditions.
|Israel said it opened the crossings in response to numerous international requests [AFP]
Karen Abu Zayd, head of the United Nations Relief and Works Agency, or Unrwa, in the territory, told Al Jazeera: "It's just a steady downward spiral here, getting worse all the time. What we had last year, we have less of this year.
"What we all hope for is that people will get together, speak together and negotiate and find a truce.
"We have made a rule to ourselves not to call it a crisis or a catastrophe or a disaster, because the situation is getting worse and worse, and what are we going to call it tomorrow?"
Israel's cabinet planned on Sunday to debate a decison by a security panel to hit back at fighters, beginning with air raids on Hamas targets, political sources said.
Israel withdrew its forces and settlers from Gaza in 2005. A military offensive could involve ground combat likely to result in high casualties.
Tzipi Livni, the Israeli foreign minister, discussed the crisis on Thursday with Hosni Mubarak, the president of Egypt, which borders Gaza to the west and which brokered the truce in June.
Mubarak urged restraint on both sides.
Ehud Olmert, the Israeli prime minister, has said he does not wish to reoccupy the coastal strip.
At the same time, he appeared on an Arabic television channel, urging Gazans to reject Hamas and stop the rocket attacks.
Olmert said it was a last-minute appeal and said he would not hesitate to use Israel's military might if they did not.