In one major attack on Wednesday, Israeli warplanes rained fire near a street market in the Shijaiyah neighborhood of Gaza City, killing at least 17 people and wounding another 200, according to medics.

The attack took place while Israel was observing a unilateral four-hour humanitarian lull, which went into effect at 1200 UTC. However, Israel said the truce would not apply in places where troops were "currently operating."

The Islamist group Hamas, which dominates the Gaza Strip, denounced the four-hour pause as a publicity stunt.

The Israeli army said three of its soldiers were also killed on Wednesday. That brings the overall numbers of soldiers killed to 56 since Israel began its offensive on the Gaza Strip on July 8 in a declared bid to put a stop to militant rocket fire at Israeli territory.

The army said more than 50 rockets were fired from Gaza into Israel on Wednesday, causing no reported casualites or damage.

'Unjustifiable attack'

The Shijaiyah attack came just hours after Israeli tank shells allegedly hit a UN-run school in the Jabalya refugee camp, killing at least 15 Palestinians who were sheltering there. It was the second time in a week that a UN school housing refugees has been hit in a week.

Israel says it had responded to enemy fire coming from near the school, where some 3,300 Palestinians had taken refuge. The United Nations Relief and Works Agency on Tuesday said it had found a cache of rockets concealed at another Gaza school - the third such case since the conflict began - and condemned unnamed militant groups for putting civilians at risk.

UN Secretary General Ban Ki-moon has, however, condemned the attack as "unjustifiable", and called for those responsible to be held to account.

Ban, who accused the Israeli military of ignoring repeated communications on the school's location, called the incident a "reprehensible attack" that demanded "accountability and justice."

The UNRWA's commissioner-general, Pierre Krähenbühl, called the attack a "serious violation of international law by Israeli forces," saying preliminary investigations suggested at least three shells hit the school where 3,300 people were sheltering.

"These are people who were instructed to leave their homes by the Israeli army," Krähenbühl said.

The United States has also condemned the attack.

Uncompromising stances

The Gaza Health Ministry says 1,323 Palestinians - most of them civilians - have been killed since Israel began its offensive 23 days ago. On the Israeli side, three civilians have been killed in addition to the 56 military deaths.

International efforts to put a stop to the conflict have so far ended in failure.

Hamas has said there can be no ceasefire unless Israel lifts its eight-year blockade on Gaza, while Israel has insisted it must first be allowed to destroy tunnels used by Hamas militants for cross-border attacks, with neither side prepared to accept the other's conditions.

tj/rc (AFP, Reuters, dpa)