GAZA CITY– Palestinian medical officials say the death toll from an Israeli airstrike outside a United Nations school in the Gaza Strip has risen to 30.

　　A rocket fired by Palestinian militants at southern Israel leaves a thin trail of white smoke, as smoke caused by explosions from Israeli forces' operations rises from buildings on the outskirts of Gaza City, Tuesday, Jan. 6, 2009. [Agencies]

　　The attack occurred about 10 yards (meters) outside the school in northern Gaza. It was the second deadly Israeli attack to strike a UN school in the past few hours.

　  Hospital director Bassam Abu Warda confirmed the 30 deaths from the second airstrike.

　　In both cases, the schools had been used as shelters for people displaced by Israel's offensive against Hamas militants. More than 500 people have been killed in the 11-day operation, including many civilians.

　　A top UN humanitarian official has condemned the violence and demanded an investigation.

　　Israel isn't commenting. But it accuses Hamas of using schools, mosques and residential areas for cover.