Israeli Prime Minister Ehud Olmert says the Israeli army shelled the U.N. complex in Gaza City today because, he told reporters, soldiers "were attacked from there and the response was harsh."
The shelling set a U.N. warehouse full of food and supplies intended for Palestinians on fire.
Olmert's office confirmed to ABC News that Olmert talked with U.N. Secretary-General Ban Ki-moon and told him, "This was a sad incident for which I am sorry but our troops were attacked from there and the reaction was forceful."
The United Nations had already dismissed the self-defense charge.
"There were no militants in UNRWA [U.N. Relief and Works Agency ] compound at the time," Johan Eriksson, a U.N. spokesman, told ABC News earlier today.
Also today Israeli Defense Force officials told ABC News that Hamas Interior Minister Said Siam was killed during an Israeli airstrike on his brothers home in Gaza.
Hamas television reported that Siam was killed in what it says was an air strike that flattened a home in Gaza City. A top aide, Siam's brother and members of his brother's family were also killed, they reported.
Siam is considered to be among Hamas' top five leaders in Gaza.
U.N. representatives also claimed the rockets that struck their compound today contained white phosphorous, which is not intended for use on populated targets.
Ban, who is in Israel today trying to broker a cease-fire, called the attack an "outrage" and has demanded an investigation.
Speaking from Tel Aviv Ban told reporters, "I conveyed my strong protest and outrage to the defense minister and the foreign minister and demanded a full explanation."
Prime Minister Gordon Brown condemned Israel's shelling of the United Nations' HQ in Gaza today as "indefensible".
"When the United Nations is doing such vital work - humanitarian work amongst women and children in Gaza no one can defend this attack ," Brown told reporters.
"I hope these events will convince everybody that an immediate ceasefire is absolutely essential," he added.
According to the UN the Israelis knew what they were hitting, "this attack is unforgivable. The IDF [Israel Defense Forces] confirmed to us they have the GPS [Global Positioning System] coordinates of all our buildings in Gaza," Eriksson said.
Only three people were injured in the attack although there were 700 sheltered in the compound.
"Moreover, the Israeli army was notified that we have several fuel tankers in the compound that we intended to move to a safer location today, " Eriksson added.
The fuel from these tankers could explode at any time. Television pictures from the compound showed the U.N. warehouse engulfed by flames and stockpiles of food literally going up in smoke.
Since this conflict began Israel has repeatedly stressed Hamas' reported use of human shields, saying the militants' tactic of firing from civilian areas leads to Palestinian casualties.
In a previous attack on a U.N. school, in which 42 people died, the Israeli army claimed that Hamas fighters had been using the school as a base from which to launch rockets.
Ban told reporters today, "The [Israeli] defense minister said to me it was a grave mistake and he took it very seriously. He assured me that extra attention will be paid to U.N. facilities and staff and this will not be repeated."
At the same news conference Israeli Foreign Minister Tzipi Livni defended the Israeli army's actions.