Palestinian boys play soccer after attending school in Jebaliya in the northern Gaza Strip on Saturday. (Anja Niedringhaus/Associated Press)

Schools operated by the United Nations in the Gaza Strip reopened Saturday for the first time since the start of a ceasefire between Israeli forces and Hamas militants.

About 200,000 children study at the UN schools, many of which were closed after Israel launched its offensive in late December.

Mortar shells struck a number of UN facilities during the offensive, including a school in the town of Jebalia, where nearly 40 Palestinians who had sought refuge from the fighting were killed.

Israel waged a 22-day war meant, it said, to end rocket fire on southern Israel from Hamas-ruled Gaza. The three-week onslaught killed 1,285 Palestinians, mostly civilians, according to the Palestinian Centre for Human Rights.

A Palestinian flag and a sign bearing the name of Muhammed Elabsi, killed during the recent Israeli operation in the Gaza Strip, are displayed at his desk at Rafah Primary School for Boys in Rafah refugee camp in the southern Gaza Strip on Saturday. (Khaled Omar/Associated Press)

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

On Saturday, representatives from several aid agencies toured neighbourhoods that were destroyed and began talking to some of the thousands of people who have lost homes.

The reopening of schools began a week after a tentative ceasefire marked a small step back to normalcy for Gaza's 1.4 million residents.

U.S. President Barack Obama's special envoy to the Middle East is to arrive in the region sometime next week to begin the new administration's efforts for Israeli-Palestinian peace.

Former senator George Mitchell is expected to visit both Israel and the occupied West Bank, Western diplomats said on Saturday.

With files from the Associated Press