Thousands of children returned to school in the Gaza Strip Saturday, seven days after Israel ceased its military operations in the Palestinian territory.

In addition to Gaza's public schools, run by Hamas since the organization took over the territory in 2007, schools operated by the United Nations also re-opened to about 200,000 children.

The return to school was a sign that life in Gaza was returning to normal after Israel's three-week air and ground assault against Hamas targets.

The offensive, which was launched on Dec. 27 to stop rocket fire from Hamas militants into southern Israel, killed 1,285 Palestinians, including 280 children, according to the Palestinian Centre for Human Rights.

Thirteen Israelis, including three civilians, were also killed during the fighting, according to the Israeli government.

The fighting damaged more than 30 UN schools, which were also used as makeshift shelters by thousands of people in Gaza seeking refuge from the clashes between Hamas militants and Israeli soldiers.

At the UN's Fakhoura Elementary school in the northern Gaza town of Jebalia, three chairs bore the names of three students killed during the offensive.

Local witnesses said that Palestinian militants fired rockets from next to the school, where hundreds of locals took shelter from the fighting.

The Israeli military responded by firing mortars that landed near the school, killing about 40 people, mostly civilians, Palestinian health officials said.

The shelling of UN buildings has strained relations between the international organization and the Israeli government.

A total of 50 UN buildings have been damaged in the fighting, including the organization's main compound in Gaza City.

According to UN officials, that building was hit by white phosphorous shells, which set a food warehouse on fire.

The Israeli military has said it will conduct an investigation into whether troops fired phosphorous shells, which are used to create smoke screens and light up the night, into civilian areas.

With files from The Associated Press