One of the Israeli strikes hit a United Nations compound that was being used to shelter up to 700 refugees from the attacks. U.N. Secretary General Ban ki-Moon, who is in the Middle East working to negotiate a ceasefire, called the attack "an outrage."
"I conveyed my strong protest and outrage to the defense minister and foreign minister and demanded a full explanation," Ban said.
Israeli Prime Minister Ehud Olmert defended the attack, saying Thursday that it was a response to a Palestinian attack on Israeli troops that originated from the United Nations Relief and Works Agency building, Reuters reported.
"It is absolutely true that we were attacked from that place," Olmert said in broadcast remarks. "But the consequences were very sad and I apologize for it."
Three staff members were injured in the U.N. compound. More than 1,000 Palestinians and 13 Israelis have been killed since the conflict began on Dec. 27.
Israeli ground troops have focused on Tel al-Hawa neighborhood in Gaza City. Many residents have fled, searching for safety as Israeli forces enter heavily populated areas.
Israeli Foreign Minister Tzipi Livni accused Hamas of using civilians as shields. "And at the end of the day, even though we try to avoid civilian casualties, these things happen," Livni said.
Livni is running to replace Olmert as prime minister in the country's Feb. 10 election.
The heavy shelling that hit the compound as well as a media building and a hospital may be Israel's final push in Gaza Strip before the two sides begin to finalize a ceasefire deal at talks in Cairo.
An Israeli envoy joined Egyptian mediators on Thursday to discuss an Egyptian-brokered truce for a phased cease-fire.
Israel has demanded that Hamas stop firing rockets into southern Israel and end weapons smuggling through Egypt into Gaza. Israeli officials say they will not agree to a deal that allows Hamas to rearm.
Israeli analyst Gerald Steinberg of Bar Ilan University told National Public Radio that the Israeli leadership would like to end the conflict before President-elect Obama's inauguration on Jan. 20.
"It would be good to have a new page when, a semi-new page, nothing's new in the Middle East, but if you have the pictures of the inauguration combined with a lot more fighting in Gaza and outside, that would take some of the sheen off the Obama inauguration. So Israel's very conscious of that," Steinberg said.
Hamas rocket fire continued on Thursday as they launched about 14 rockets over the border. According to Reuters, police said they caused some damage but no casualties. Rocket attacks have decreased since the conflict began.