Israeli fuel sanctions against the Hamas-run Gaza Strip punish an entire population and are unacceptable, UN Secretary General Ban Ki-moon has said.
He spoke after Israel began reducing petrol and diesel supplies in line with its declaration last month that the Strip is a hostile entity.
Hamas seized control of the Strip in June from its Palestinian rivals Fatah.
The European Union also voiced concern at the Israeli cuts, calling them "collective punishment".
BBC producer Rushdi Abu Alouf in Gaza says that the cuts in supply are not being felt by Gazans yet.
Israel says the supply cuts are a non-violent way of increasing pressure on Hamas, and follow continued rocket attacks from Gaza.
An Israeli official, Shlomo Dror, said oil supplies were being cut by up to 15% over the coming week but there would be no reduction in supply to Gaza's power station.
Israel says there will be enough fuel for hospitals.
Israeli and Palestinian human rights groups have filed a petition at the Israeli supreme court in an attempt to halt the cuts.
In a statement read out by a spokesperson, Mr Ban urged Palestinian militants to end indiscriminate rocket attacks on Israel, which he condemned.
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.
The move has 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. ( BBC )