The siege of Gaza has reached a vicious new intensity. Last Thursday, Israel blocked the delivery of fuel oil supplies to the Strip. The result is that Gaza's only power station has not functioned since the weekend. Hundred of thousands of homes in the territory have been left without power. Hospitals have been forced to rely on diesel generators. Bakeries and petrol stations have closed. International aid organisations working in the Strip have warned of a threat to sewage and water supplies if the blockade continues.

The Israeli government denies it is creating a humanitarian crisis and calls the blackout a ploy by Hamas, the authority in Gaza, "to attract international sympathy". This is not a view shared by John Ging, the operations director for the United Nations refugee agency in Gaza. It is true that the power station normally supplies only a third of the Strip's electricity, but it is also clear that the Strip's energy infrastructure cannot cope with such a massive disruption to supplies. The suffering in Gaza is real, and is the result of Israel's embargo.

While Israel is reluctant to admit the effects of its blockade, it makes no bones about what the siege is intended to achieve: it is designed to pressure Hamas into putting a stop to the rocket attacks being launched against Israel from within Palestinian territory. Israel has a right to attempt to stop the attacks on its civilian population, but not by any means. International law specifically forbids collective punishment of occupied populations. The Geneva Conventions stipulate that occupying powers have an obligation to supply utilities such as water and power to occupied populations.

Israel has attempted to get around this by arguing that it is no longer bound by the law governing the administration of occupied territories because it withdrew its troops from Gaza in 2005. But that is thoroughly unconvincing. Israel still controls Gaza's borders, airspace and territorial waters. It may have begun referring to the Strip as a "hostile entity", but this is plainly an area still under Israeli control.

The UN Secretary-General, Ban Ki-Moon, and the European Union's External Affairs Commissioner, Benita Ferrero-Waldner, have both declared the embargo unacceptable. But, as ever in the Israeli-Palestinian conflict, events are being propelled by internal dynamics. There has been a rise in the number of rocket attacks from Gaza of late. More than 200 rockets and mortar bombs have pounded southern Israel in the past week. The Israeli Prime Minister, Ehud Olmert, has been under domestic pressure to respond to the bombardment. The embargo is his answer, along with an escalation in targeted killings of suspected Palestinian militants in the Strip. Israel's Deputy Prime Minister, Haim Ramon, claims the strategy is working because the number of rockets being fired has fallen. But it is far too early to come to such a judgement. The likelihood is that, in the absence of a negotiated ceasefire between Israel and Hamas, the rockets will soon begin to fall again.

The cycle needs to be broken somehow. But Mr Olmert is not making decisions from a position of strength. The resignation of the hardline leader of the Yisrael Beiteinu party, Avigdor Lieberman, from Israel's coalition government, has left Mr Olmert with only a slim majority in the Knesset. There is a danger that he will stretch out the embargo to counter accusations of being "weak" in the face of Israel's enemies.

Israel is justified in taking the rocket attacks seriously. But collective punishment of 1.5 million Gazans is no way to deal with the threat. All this blockage will do is drive more Gazans into the arms of militants and entrench a hatred of Israel among them. Rather than making Israel safer, it will merely expose her to greater danger.