Israel's military onslaught against Hamas has aggravated Gaza's humanitarian crisis, with electricity and communications cut and the population now facing dire food shortages, aid agencies said Sunday.

The Israeli army said the World Food Programme halted emergency shipments to Gaza because its warehouses are full but the UN agency insisted it was desperate to get supplies into the enclave.

"The military incursion compounds the humanitarian crisis following more than a week of shelling and an 18-month long blockade of the territory," the UN humanitarian coordinatory said in a daily report.

There was an "almost total blackout" across most of Gaza and land and mobile phone networks were also down because they depend on backup generators which had no fuel, the report said.

All Gaza City hospitals have been without mains electricity for 48 hours and now rely on backup generators which the UN said were "close to collapse."

The report said that "for the second consecutive day Israeli authorities have refused to allow an ICRC (International Committee of the Red Cross) emergency medical team into Gaza" to help at the main Shifa hospital. The territory has been sealed off for more than two days.

At the hospital the breakdown of temporary generators would threaten 70 patients linked to machines in the intensive care unit, including 30 infants.

More than 510 Palestinians have already been killed in Israel's nine day old offensive on Hamas targets in the Gaza Strip, which on Saturday was intensified with the launch of a massive ground operation.