Gunshots and an anti-tank missile were fired at IDF troops near the UN compound that was attacked by the IDF on Thursday, senior defense officials told The Jerusalem Post.
According to the officials, the IDF responded by firing artillery shells at the location of the gunmen, causing damage to the UN installations. At least three people were wounded in the attack and the building was set on fire.
The IDF's Gaza Coordination and Liaison Administration coordinated the arrival of five fire trucks to the compound to help put out the flames.
UN Secretary-General Ban Ki-moon, who was in Israel on 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."
Palestinians reported that an IDF tank shell also struck one of the wings of the Gaza Al-Quds Hospital midday Thursday. Witnesses said part of the structure was on fire. The army said that the building was being used by Hamas men who were firing on IDF troops.
The International Federation of Red Cross and Red Crescent Societies said the damage caused to the Al Quds hospital was "completely and utterly unacceptable based on every known standard of international humanitarian law."
The Geneva-based group said the second floor of the hospital took a direct hit that caused fires in the pharmacy and severe damage in many parts of the building.
Its sister organization, the International Committe of the Red Cross, also issued a statement Thursday saying the shelling put the lives of patients and medical staff at risk.
Officials contacted at the Israeli mission in Geneva late Thursday could not immediately comment on the issue.
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 as Israel ground forces were reportedly moving deeper into Gaza City. 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.
Earlier, the IDF observed 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 25 rockets were fired at the western Negev by evening on Thursday.
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.
Elana Kirsh and JPost.com staff contributed to this report.