A top United Nations humanitarian official on Tuesday called on Israel to immediately open up crossing points into Gaza for full access to relief aid following its devastating three-week offensive against Hamas militants.

"Israel has a particular responsibility as the occupying power in this context, because of its control of Gaza's borders with Israel, to respect the relevant provisions of international humanitarian law," UN Under-Secretary-General for Humanitarian Affairs John Holmes told the Security Council in a report on his just-completed visit to Gaza, Israel and the Palestinian Authority.

"It is therefore critical that new steps are taken immediately by the Israeli authorities to move to the sustained re-opening of crossing points," he said, stressing that improving the living conditions of Gaza's 1.5 million people was vital to avoid further despair and undermining the two-state diplomatic solution to the decades-old Middle East conflict.

As he did frequently during the assault Israel launched on December 27, 2008, with the stated aim of ending Hamas rocket attacks from Gaza, Holmes meted out blame to both sides in the conflict.

"The reckless and cynical use of civilian installations by Hamas, and the indiscriminate firing of rockets against civilian populations, are clear violations of international humanitarian law," he said. However, even taking into account Israel's security concern to protect its own civilian population, it is clear that there are major questions to be asked about the failure of the Israeli Defense Force (IDF) to protect effectively civilians and humanitarian workers in Gaza.

"Given the scale and nature of the damage and loss of life, there are also obvious concerns about a lack of wider respect for international humanitarian law, particularly the principles of distinction and proportionality. There must be accountability," he said.

Holmes cited the toll of the conflict: some 1,300 Palestinians killed, and more than 5,300 injured, 34 percent of them children, according to Palestinian Ministry of Health figures that have not been seriously challenged; 21,000 homes destroyed or badly damaged; over 50,000 people displaced in UN Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) structures during the height of the fighting, with tens of thousands more sheltering with families and friends.

"Widespread destruction was caused to Gaza's economic and civil infrastructure," he said. "I saw for example, an entire industrial and residential area in East Jabalia which had been systematically bulldozed, an area of at least one square kilometer; one of the best schools in Gaza reduced to rubble; and much of the Al Quds hospital in Gaza City burned out."

But he stressed the critical need to look forward to bring urgent relief to Gaza after 18 months of closure, which steadily weakened health, livelihoods and infrastructure even before the recent offensive.

"A massive humanitarian effort is now needed in areas such as food security, nutrition, water and sanitation, shelter, essential repairs of power, roads and other basic infrastructure, rebuilding the health system, rubble removal, unexploded ordnance and psychosocial care. As only one example, 1.3 million Gazans, almost 90 percent of the population, now need food aid," he said, noting that he would launch a Flash Appeal on February 2.

Much freer access for goods and staff is needed, Holmes, who also serves as UN emergency relief coordinator, declared. While Israel has allowed increased shipments of basic commodities with 120 truckloads getting in on good days, the normal daily requirement is a minimum of 500. Many humanitarian workers continue to be refused regular entry.

If aid workers continue to face rigid limits on their movement and essential items such as construction materials, pipes, electrical wires and transformers are kept out, the lives of the Gazan people cannot significantly improve, he said.