Much of Gaza's public infrastructure has been destroyed and the territory is in a "critical emergency" after seven days of devastating bombing with air strikes averaging one every 20 minutes, the UN said yesterday.

Israel yesterday targeted the homes of more than a dozen Hamas figures and destroyed a mosque in Gaza. A young boy was killed by an Israeli bomb east of Gaza City and three children died in the southern town of Khan Yunis. The military said it hit 35 targets, including homes, smuggling tunnels and buildings storing weapons.

Despite Israel's insistence that there was no humanitarian crisis, senior UN officials in Jerusalem said there were shortages of food, water, medical supplies and even cash.

"By any definition this is a humanitarian crisis and more," said Max Gaylard, the UN humanitarian co-ordinator for the Palestinians. He said there had been on average one air strike every 20 minutes since the bombing began, intensifying at night and covering the whole Gaza Strip.

The death toll in Gaza climbed to more than 400 dead and at least 1,700 injured. The UN said it believed a quarter of the dead were civilians. Palestinian militants in Gaza continued to fire rockets into southern Israel, where four people have been killed in the week since Israel's bombing began.

Israeli officials briefly opened the Erez border crossing to allow out from Gaza about 300 Palestinians with foreign passports. Foreign journalists were still banned from entry.

Israeli tanks, troops and armoured vehicles remain in position on the border ready for a possible ground offensive, although there was no indication when that might start or how large it might be.

Robert Serry, the UN special co-ordinator for the Middle East, blamed the conflict on "the irresponsibility of Hamas rocket attacks and the excessiveness of Israel's response".

He said a ceasefire was needed to halt the violence but added: "We must create new conditions on the ground to ensure this does not happen again."

Serry said the Palestinian Authority, now run by Fatah, would have to take control of the crossings in Gaza, even though the strip is under the control of Hamas and the factions are bitter rivals. Asked if there would in future be international peacekeepers or monitors in Gaza, he said: "We have to look into possibilities like an international presence."

Gaza's sole power plant has not worked since Monday because Israel has halted deliveries of industrial diesel. Hospitals are overwhelmed and there is not enough wheat grain for UN food deliveries, UN officials said.

A cash shortage in Gaza meant the UN could not continue cash assistance to 95,000 of the very poor. It could only meet half its December staff salary payments and was reduced to bartering with flour in some instances.

In Israel, the conflict has improved the political fortunes of Ehud Barak, the defence minister, whose Labour party has climbed in opinion polls this week to a stronger third position.

The foreign minister, Tzipi Livni, of the Kadima party, has now drawn level in the polls with Binyamin Netanyahu, the Likud opposition leader, who had been a clear favourite to win February's general elections.