The IDF on Thursday afternoon said that the army ordered an artillery shelling of the UNRWA building in Gaza only after a Givati Brigade force came under anti-tank fire from within the building. Channel 10 reported that artillery pieces situated outside the Strip pounded the building.

Day 20 UN spokesman Chris Gunness said at least three people were wounded in the shelling. The building was reportedly on fire. Speaking to Sky News, Gunness called for an inquiry into the incident.

UN Secretary-General Ban Ki-moon, in Israel Thursday to promote a cease-fire between Israel and Hamas, expressed "strong protest and outrage" at the reported shelling of the UN compound.

Ban also demanded an investigation into the shelling, and said Defense Minister Ehud Barak had told him it was a "grave mistake."

IDF: Civilian deaths less than 25% of total
The IDF coordinated the arrival of five fire-trucks at the building to extinguish the blaze.

Palestinians reported that an IDF tank shell also struck one of the wings of a Gaza hospital midday Thursday. Witnesses said part of the structure was on fire. The army said that the building was also being used by Hamas men who were firing on IDF troops.

In total, IAF aircraft struck some 35 targets in the Strip throughout the morning, including armed cells and rocket launch positions.

IDF troops also opened fire at two UN vehicles in the Strip on Thursday. One of the cars had no markings identifying it as belonging to the organization. The other was marked as a UN car, but gunshots were fired from it at IDF troops, who returned fire.

On Thursday morning, IDF tanks fired shells at three high-rise buildings in the Tel Hawwa neighborhood of Gaza City, according to Palestinian witnesses.

Palestinian reports said that the IDF hit a multi-story media building that houses several media outlets, including Reuters, Al Arabiya and the BBC. Witnesses said that a Qatari journalist was wounded and that the building was evacuated.

Defense officials said Hamas operatives had barricaded themselves inside the press office in Gaza, and were using people there as human shields. There were 23 people inside, said the officials.

As IAF aircraft hammered targets in Gaza City, ground troops advanced into a crowded residential area on the city's outskirts. Thousands of residents were reportedly fleeing their homes. The targeted buildings are about 1.5 kilometers from the ground forces.

The move marked the deepest thrust into the city in the twenty days of Operation Cast Lead.

Meanwhile, the IDF was observing a daily "humanitarian corridor" between 11 a.m. and 3 p.m., to allow for the transfer of humanitarian aid into the Strip, and for Gazans to stock up on necessary supplies.

In a statement released Thursday morning, Defense Ministry spokesman Peter Lerner said that a total of 1,136 trucks of humanitarian supplies have been transferred to Gazans since the beginning of the IDF operation in the Strip, as well as fuel trucks and medical personnel.

The statement added that the electrical grid in the Gaza Strip was functioning at 74% capacity, up from 40% at the beginning of the operation.

Lerner said the state of the electrical supply was "a direct result of the coordination between the IDF, the Palestinian Energy Authority and the International Committee of the Red Cross."

In previous days, Hamas attacks on the South have continued during these pauses in operations.

While the number of rockets fired has dropped dramatically in recent day, at least 22 rockets were fired at the western Negev on Thursday morning.

Overnight Wednesday, IAF aircraft struck some 70 targets, including weapons positions, rocket squads and a mosque in southern Gaza that it said served as a rockets arsenal, according to the IDF. Palestinian medical officials said seven people were killed in the strikes. Four of those killed were in the southern strip and three in Gaza City, according to Army Radio.

Also Wednesday overnight, the IAF attacked five armed Palestinians who turned out to be the personal bodyguards of senior Hamas official Mahmoud Al-Zahar. The five men were reportedly guarding Zahar's house in southern Gaza City and were all killed, according to reports.

Eleven IDF soldiers were wounded overnight Wednesday in clashes with Hamas gunmen.