Israel began cutting vital fuel shipments to the Gaza Strip on Sunday, following through on a promise to step up pressure on the area's Hamas rulers in response to months of Palestinian rocket attacks.

Dor Alon, the Israeli energy company that sells fuel to Gaza, confirmed it had received instructions from the Israeli Defense Ministry to reduce shipments. The move drew harsh condemnation from Palestinians in Gaza, which relies on Israel for almost all its fuel and gasoline and more than half of its electricity.

"This is a serious warning to the people of the Gaza Strip. Their lives are now in danger," said Ahmed Ali, deputy director of Gaza's Petroleum Authority, which distributes Israeli fuel shipments to private Palestinian companies. "The hospitals, water pumping station and sewage will now be affected by the lack of fuel."

Ali said daily fuel shipments on Sunday were more than 30 percent below normal.

He said it would take several days for the fuel crunch to be felt, since Gaza keeps about four days of fuel in reserve. But truck drivers at Gaza's main fuel depot complained that they were unable to fill their tankers, and some drivers said they were turned away altogether.

The fuel crunch is expected to hit Gaza harder than a planned cuts in electricity, because the area already suffers frequent power outages.

Israeli Defense Minister Ehud Barak approved a plan to begin cutting fuel supplies and electricity to Gaza. The government last month declared Gaza a "hostile entity" and approved the plan for cutoffs as a response to the near-daily rocket attacks carried out by Palestinian militants in Gaza.

Israel says it holds the Hamas militant group, which seized control of Gaza in June, responsible for the continued rocket fire. Although smaller militant groups have carried out most of the rocket attacks, Hamas has done little to stop them.

The plan has drawn criticism from human rights groups and the international community, which say the sanctions amount to collective punishment.

In a radio interview Saturday, Deputy Defense Minister Matan Vilnai insisted the fuel and electricity cutoff is not a response to the rocket fire. Instead, he said the move was the latest step by Israel to "disengage" from Gaza following its withdrawal of all troops and settlers from the area two years ago.

"This is the continuation of our disengagement, since the troops pulled out," he told Israel Radio. "This is not connected to Qassams (rockets)," he added. "It is a deeper, broader disengagement."

He said the sanctions are meant to wean Gaza's dependence on Israel and conceded they are unlikely to halt rocket fire.

Palestinians and Israeli human rights groups say that Israel remains responsible for the well-being of Gaza's 1.4 million people. They say that despite the 2005 withdrawal from Gaza, Israel still controls the area's borders — limiting imports, exports and the movement of human beings in and out of the area — and therefore continues to occupy the strip.