"There are certainly multiple dead and injured," U.N. Relief and Works Agency director in Gaza Robert Turner told Al Jazeera. He added there was no advance warning of the missile that struck the facility -- a school adapted to house families displaced by several weeks of Israeli shelling of Gaza.
According to the Gaza health ministry, over 15 people were killed, and 200 other were injured.
The incident comes as the United Nations continued to press for a cease-fire in a conflict that has seen significant civilian casualties and a growing humanitarian crisis. Valerie Amos, U.N. humanitarian affairs and emergency relief undersecretary, called conditions in Gaza "dire" Thursday, noting civilians were running out of food and water.
The 17-day conflict, which includes reciprocal shelling by the Israeli Defense Force and Hamas forces from Gaza, as well as a ground offensive Israel launched a week ago, has brought the death toll to 747 Palestinians in Gaza. Additionally, 34 Israelis have been killed, 32 of whom took part in the land offensive.
The comments by Amos came a day after Navi Pillay, U.N. high commissioner for human rights, suggested "a strong possibility that international law had been violated" by Israel's airstrikes, specifically mentioning attacks on homes and hospitals in Gaza. Israel has claimed U.N. schools have been used to hide soldiers and weapons.
Turner, referring to the incident Thursday, said the UNRWA was not aware of any weapons or missiles in the shelter.