The United States has said there is "little doubt" Israeli bombs killed 19 people at a UN shelter in Gaza, in its strongest condemnation yet of Israeli attacks on civilians in the Palestinian enclave.

Josh Earnest, a White House spokesman, said on Thursday that the shelling of the school in Jabaliya was "totally indefensible" and called on Israeli forces to "live up to their own standards" of protecting civilians.

Earnest said: "While we underscore the importance of a full and prompt investigation of this tragic incident, as well as a shelling of other UN facilities and schools that have been hit, it does not appear there's a lot of doubt about whose artillery was involved in this incident."

"The shelling of a UN facility that is housing innocent civilians who are fleeing violence is totally unacceptable and totally indefensible.

"We believe the Israeli government and the Israeli military need to do more to live up to the own standards that they have set for protecting innocent civilians," Earnest said.

His comments came after UNRWA, the UN's refugee agency in Gaza, said its investigations showed that Israeli fire was responsible for the attack, which it called shameful and an affront to humanity.

Al Jazeera sources have meanwhile learned that the US and UN were preparing to announce a new plan for a 72-hour truce in Gaza.

It was not clear if Israel or Palestinian representatives including Hamas have agreed to the plan, which is due to be presented by the US secretary of state, John Kerry, and the UN's secretary general, Ban Ki Moon in Cairo.

Al Jazeera's diplomatic editor James Bays said the diplomats were "trying to get ahead of everyone" and "shame everyone who does not abide by the proposal".

"This is a bold move, an importatnt move," our correspondent said.

Earlier on Thursday, the Israeli prime minister, Benjamin Netanyahu, said his country was determined, regardless of ceasefire efforts, to complete the destruction of tunnels that Palestinian fighters have built under the Gaza-Israel border.

"Until now, we have destroyed dozens of terror tunnels and we are determined to finish this mission - with or without a ceasefire," Netanyahu said.

As of late Thursday, Gaza's emergency services put the toll at 1,437 dead, with more than 8,100 wounded. The latest death toll surpassed the 1,391 Palestinians killed during the Operation Cast Lead between December 27, 2008 to January 18, 2009. During that operation, 13 Israelis were also killed.

Despite the latest condemnation, the Obama administration continues to allow Israel to tap a local US arms stockpile to replenish grenade and mortar stocks depleted during its offensive, US officials said.

The US has also announced on July 28 that it will double its invesment on Israel's Iron Dome system against incoming rockets, and an additonal $225m in 2014 to accelerate Iron Dome components.