Israeli energy sanctions against the Hamas-run Gaza Strip punish an entire population and are unacceptable, UN Secretary General Ban Ki-moon has said.
Israel insists supplies to Gaza's power station will continue
The EU also voiced concern after Israel began reducing petrol and diesel supplies in response to militant rocket attacks on its territory.
Hamas seized control of the Strip in June from its Palestinian rivals Fatah.
Israel's attorney-general is seeking a halt to electricity cuts pending an assessment of their likely impact.
Gaza relies on Israel for almost all its fuel and petrol, and more than half of its electricity.
Israel says fuel cuts of up to 15% are a non-violent way of increasing pressure on Hamas.
It insists there will be enough power for hospitals and that supplies will continue to Gaza's sole power station.
In a statement read out by a spokesperson, Mr Ban urged Palestinian militants to end indiscriminate rocket attacks on Israel, which he condemned.
ISRAEL'S PROPOSED SANCTIONS
Gaza relies on Israel for almost all its fuel and more than half of its electricity
Cuts of up to 15% in petrol and 10% in diesel
Targeted electricity outages of at least 15 minutes in response to each new rocket attack
Supplies of crude diesel to Gaza's power plant not due to be affected
Source: Israeli officials
But he also stated his belief that the "punitive measures taken by Israel... harm the well-being of the entire population of the Gaza Strip".
The cuts would, he said, "deepen the humanitarian distress" of Gaza's 1.4m residents.
Benita Ferrero-Waldner, EU commissioner for external relations, said on a visit to Jerusalem she was "very concerned" about the Israeli move though she understood Israel's "distress" over rocket attacks.
"I think collective punishment is never a solution," she said.
As part of its sanctions, Israel envisages shutting down one of its power lines to Gaza for 15 minutes after a rocket attack, with the cut-off period gradually increasing to a two-hour limit if barrages continue.
Attorney-General Menahem Mazouz said cuts in electricity supplies could not be allowed before a full assessment of the possible humanitarian consequences.
Mr Mazouz, who is also the government's legal adviser, called on security chiefs to carry out "supplementary examinations".
However, the justice ministry statement confirmed that the attorney-general had approved the decision to make fuel cuts.
Israeli and Palestinian human rights groups have filed a petition at the Israeli Supreme Court in an attempt to halt the cuts.
'A real crime'
BBC producer Rushdi Abu Alouf in Gaza says that cuts in supply are not being felt by Gazans yet.
The cuts have been condemned by Hamas, which governs the territory, and a number of international organisations.
"The Israeli decision is a real crime against 1.5 million Palestinians in Gaza," said Hamas spokesman Taher Nouno.
Rockets are fired by Palestinian militants from Gaza into Israel on an almost daily basis. Israel withdrew all its settlements from Gaza in 2005.
Palestinian militants say they are responding to continued Israeli aggression in Gaza and the West Bank.