A power plant in Gaza City has shut down, affecting 500,000 local inhabitants and forcing local hospitals to run on reserve fuel.
Large parts of the Gaza Strip, particularly Gaza City, were in darkness after the main power station shut down its generators on Saturday.
The Hamas government's energy department said that about 55 per cent of Gaza City and 35 per cent of the territory's other areas had power outages as a result of the shutdown.
With hospital generators running out of fuel, it is feared that medical equipment will stop functioning soon.
Border opened
An estimated 60 per cent of Gaza's power supply comes from its own power station and the rest from Israel.
In the past, Israel has resumed fuel supplies just hours before Gaza's stocks ran out.
As the siege of Gaza continued, Al Jazeera's Ayman Mohyeldin, reporting from the Gaza side of the Rafah crossing, said that the main border point between the Gaza Strip and Egypt had been reopened to allow Palestinians in need of medical treatment to pass through.
"This is part of an agreement that has been reached between Egypt and Hamas," he said.
"To keep the atmosphere of a possible ceasefire agreement alive, the Egyptians have told the Hamas authority they will allow for the next four days this restricted movement of people to go through and to come back but ... the situation in Gaza remains dire and it is not open to the general public."