Israel on Monday began reducing electricity supplies to the Gaza Strip, despite warnings the move could increase suffering and tensions in the Palestinian enclave.
The cut will reduce the mains power flow to Gaza to as little as two hours a day, though many businesses and the wealthy have their own generators.
The decision came after the Palestinian Authority (PA), which is based in the occupied West Bank, told Israel it would no longer foot the bill for electricity supplies to Gaza.
It raises concerns of rising tensions and a collapse of vital services in an impoverished and overcrowded territory that has been devastated by three wars with Israel since 2008.
Hamas has run Gaza since 2007, when it seized the strip in a near civil war from the Fatah party of Palestinian president Mahmud Abbas, in a dispute over general elections won by the Islamist movement.
Multiple attempts at reconciliation between Hamas and Fatah have failed, but the PA had continued to pay Israel for some electricity delivered to Gaza until this month.
Israel "began to reduce electricity flow by eight megawatts" into the enclave, Gaza's energy authority said.
The state-run Israel Electricity Corporation confirmed it had diminished power supplies "in accordance with a government directive".
Until Monday, Israel supplied 120 megawatts of electricity to Gaza a month, which made up about one quarter of the enclave's needs, with the PA paying the 11.3 million euros ($12.65 million) monthly bill.
Since the sole power station in Gaza ran out of fuel and stopped working in April, the 120 megawatts represent 80 percent of available power in the strip.
The Israel Electric Corporation said power supply would "effectively be reduced on two lines out of 10 every day, until the reduction applies to all 10 lines".
- Total collapse -
The Gaza Strip is home to some two million people, more than three-quarters of whom the United Nations says depend on humanitarian aid.
The power reductions come despite stark warnings of the humanitarian implications for Gazan civilians, who already suffer from critical shortages of power -- with most homes receiving only a few hours even before the cut.
Israeli human rights group Gisha said in a statement on Monday that by reducing supplies "Israel is knowingly aggravating an already dangerous situation in which the strip is teetering on the verge of a humanitarian crisis."
The vast majority of residents are Muslim and are currently observing the holy fasting month of Ramadan.
Robert Piper, UN humanitarian coordinator for the Palestinian territories, warned last week that the Palestinians were being "held hostage to this longstanding internal Palestinian dispute."
"A further increase in the length of blackouts is likely to lead to a total collapse of basic services, including critical functions in the health, water and sanitation sectors."
Hamas last fought Israel in 2014 and analysts have warned the power reduction could prompt the Islamist group to spark another round of conflict.
In a statement Wednesday, Hamas said Israel and Abbas were jointly responsible for the "catastrophic consequences" of the reduction.
The statement did not mention war, but called the measures "dangerous".
Hamas is considered to be a terrorist group by Israel, the European Union and the United States.