THE White House described the Israeli shelling of a UN school which killed 15 people as "totally unacceptable and totally indefensible".
A spokesman said there appeared to be little doubt "whose artillery was involved". The school in northern Gaza was sheltering civilians who had been told to leave their homes by the Israeli army.
The Palestinian Health ministry said that 149 Gazans had so far been killed by the Israeli army.
Palestinians fired more than 60 rockets at Israel. One Israeli was killed by a mortar bomb and several were injured.
The condemnation came before Israel and Hamas agreed to a three-day unconditional humanitarian ceasefire, starting at 8am local time on Friday morning, the US and UN confirmed in a statement.
Negotiations on a more durable truce in the three-week-old fighting would take place during the period it said, adding that Israeli troops would remain on the ground.
The head of the UN's agency for Palestinian refugees earlier warned the UN Security Council that if Israel continues its offensive in the Gaza Strip, it will have to reassume its responsibilities as occupying power, and provide humanitarian services to the population.
Speaking in New York, Pierre Krähenbühl said Gaza's infrastructure and health service were on the verge of collapse.
"I believe the population is facing a precipice and appeal to the international community to take the steps necessary to address this extreme situation," he said, noting that 220,000 Palestinians are now sheltering in UN facilities and that eight of his colleagues have been killed since the fighting began over three weeks ago.
Israel passed on its responsibilities as occupying power to the Palestinian Authority in 1994 as part of the Oslo accords, leaving the authority to provide services to the Gaza population.
In 2005, Israel withdrew its forces to the borders of the strip and pulled out of its settlements.
The Israeli army mobilised a further 16,000 reserve soldiers today as Benjamin Netanyahu, the Prime Minister, ruled out agreeing to a ceasefire in the war with Hamas until the army destroys the militant Islamist movement's network of tunnels penetrating Israel from the Gaza Strip.
"We have neutralised tens of terror tunnels and we are determined to complete this important mission with or without a ceasefire,'' Mr Netanyahu said at a meeting of his cabinet.
"I will not agree to any offer that does not allow the military to complete this important mission for the security of the people of Israel.''
The mobilisation appears aimed at giving the government the option of further escalating the ground operation in Gaza, though it is believed that Mr Netanyahu is reluctant to be drawn into an all-out reoccupation of the crowded coastal enclave.
This would increase the risk of Israeli casualties, heighten international criticism and raise fears about the chaos that would ensue if Hamas were dislodged from power.
Israel sent a delegation to Cairo on Wednesday for talks on a possible ceasefire but its demands and those of Hamas remain mutually exclusive.
Israel wants Hamas be disarmed, while Hamas insists it will not hold fire until Israel and Egypt lift border curbs that are crippling the Gaza economy and release Hamas prisoners recently rearrested in the West Bank by Israel.
Mr Netanyahu said that dismantling the tunnels "is just the first phase in the demilitarisation of Gaza".
Israel's military leaders say the army is within a few days of completing its tunnel mission, which was the stated reason for Israel following up 10 days of aerial bombardments with a ground offensive.
Israeli relations with the UN, which are often rocky, have reached a low point in recent days as UN Secretary-General Ban ki-moon strongly condemned shelling that caused the deaths of fifteen Palestinians among thousands sheltering at a UN school in north, Jebalya, Gaza.
"It is outrageous it is unjustifiable and it demand accountability and justice,'' Ban said.