It was another bloody day of fighting in the Gaza Strip today, as the Israeli military continued Operation Protective Edge, the main focus of which is to demolish Hamas’ network of tunnels leading into Israel.

John Huddy spoke to Jenna Lee on "The Real Story" today from the Israel-Gaza border to report on airstrikes that hit a popular market and a U.N. school.

The Israeli military says it only attacked the school after mortar shells were fired from nearby, underscoring that Hamas militants hide amongst the general population.

Watch the clip above to get a rare look inside one of Hamas' tunnels.

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An Israeli four-hour cease-fire for parts of Gaza ended Wednesday, during which Gaza militants fired 27 rockets at Israel, and Palestinians claimed a strike hit a crowded market.

Hamas dismissed the "humanitarian" window and Israel said it would not halt fire in areas where the military is searching for tunnels used by Hamas militants. 

Hamas spokesman Sami Abu Zuhri said it lacked any "value" because it excluded border areas from where Hamas wanted to evacuate the wounded.

Palestinian authorities claimed that an Israeli airstrike hit a crowded Gaza shopping area during the window, killing at least 16 people and wounding 150. The strike occurred in Shijaiyah, an area that was not included in the cease-fire. 

Gaza health ministry official Ashraf al-Kidra and witnesses said the shopping area was busy because residents thought a cease-fire was in place. The Palestinian Red Crescent confirmed the death toll. 

The Israeli military had no immediate comment and said it was investigating the report. 

Earlier Wednesday Palestinian authorities said that a United Nations school was hit by Israeli tank shells, resulting in 15 deaths, and about 90 injuries, while the Israeli military said it fired after its soldiers were targeted by mortars operating from the vicinity of the school. 

"In response, soldiers fired toward the origins of fire. And we're still reviewing the incident," the military said in a statement.

The United States condemned the shelling of the U.N. school, but did not say who was responsible. 

White House spokeswoman Bernadette Meehan said the U.S. is also "extremely concerned" that thousands of Palestinians aren't safe in the U.N.-designated shelters in Gaza. The agency is sheltering more than 200,000 people displaced by the fighting at dozens of U.N. schools in Gaza. 

Meehan said the U.S. also condemns those responsible for hiding weapons in U.N. facilities in Gaza, and the escalation shows the need for a cease-fire as soon as possible. 

Abu Hasna, the U.N. agency spokesman called for the international community to step in. 

"It's the responsibility of the world to tell us what we shall do with more than 200,000 people who are inside our schools, thinking that the U.N. flag will protect them," he said. "This incident today proves that no place is safe in Gaza."