Gaza's one-and-a-half million residents have been struggling to cope without electricity and other basic necessities on the fourth day of an Israeli blockade.

As hospital began to run short of fuel for generators and sewage spilled out onto the streets, Israel's prime minister said he would "not allow a humanitarian crisis in Gaza".

On Monday evening, Israel said that it would allow some deliveries of fuel and medicine into the territory on Tuesday.

"We think Hamas got the message. As we have seen in the past couple of days, when they want to stop the rockets, they can," Arye Mekel, Israeli foreign ministry spokesman, said.

Five rockets were fired from Gaza on Sunday, according to the Israeli army, down from 53 in the two previous days.

Gaza blackout

Gaza City was plunged into darkness on Sunday after its sole power plant was switched off as fuel supplies dried up following the Israeli blockade.

The decision by Ehud Barak, Israeli defense minister, will allow the European Union to resume deliveries of industrial fuel as well as diesel for generators and gas used for cooking.

Restrictions will remain in place on petrol for vehicles.

The Israeli move came after the UN agency which supports the Palestinians, warned it might be forced to stop distributing food aid to 860,000 people in Gaza because of a shortage of nylon for plastic bags and fuel for vehicles and generators.

Christopher Gunness, a spokesman for the United Nations Relief and Works Agency (UNRWA), said that the on-off blockade made it very difficult for his agency to provide relief.

"I suppose we have to be grateful for whatever they allow in, because at least it allows us to partially fulfill our mandates," he told Al Jazeera.

"This hand on the tap, allowing drips in now and drips in later, that's not what we need. What we need is a sustained opening of the crossings."

Grave situation

Margaret Chan, director-general of the World Health Organization, described the situation as grave.

"Disruptions in the continuity of essential services take a heavy toll on people needing emergency care and those suffering from conditions such as cancer, heart disease, and diabetes," she said.

EU officials said hospitals had already been badly hit in the absence of electricity.

 Khaled Radi, a spokesman for the ministry of health in Gaza, said that many of them were only performing emergency surgery.

Al Jazeera's Jacky Rowland, reporting from the east of the Gaza Strip, said raw sewage was spilling out into the streets, homes and fields because sewage treatment plants were no longer operating.

"Normally a pump would pump the sewage down the process line to a treatment plant ... but because there is no electricity the pump is standing still and as the sewage builds up it is flowing into the street," she said.

"This is a looming public health crisis."

Ehud Olmert, the Israeli prime minister, promised earlier on Monday that humanitarian aid would reach "hospitals, clinics, young children and helpless people", but did not say when.

'Murderous regime'

However, he said that he had no intentions of letting Gazans "live comfortable and pleasant lives" until rocket attacks from the territory were halted.

"As far as I'm concerned, all the residents of Gaza can walk and have no fuel for their cars, because they have a murderous terrorist regime that doesn't allow people in the south of Israel to live in peace," he said.

Israeli officials accused the Hamas movement, which controls the Gaza Strip, of exaggerating the crisis in the territory.

"While the fuel supply for Gaza has indeed been reduced ... the diversion of this fuel from domestic power generators to other uses is wholly a Hamas decision," the Israeli foreign ministry said in a statement to Al Jazeera.

"Noteworthy is the fact that while the Gaza population remains in the dark, the fuel generating power to the Hamas rocket manufacturing industry continues to flow unabated," it said.

PHOTO CAPTION

Gaza's only power plant