An Israeli tank shell struck UN's headquarters in the Gaza Strip as Egypt pressed ahead with international efforts to end the Israeli offensive that continued into a 21st day on Friday.

The shell hit the UN refugee agency's main Gaza compound Thursday after UN chief Ban Ki-moon arrived in Israel to push for a ceasefire in Gaza. The shell injured three employees and set a warehouse on fire.

Ban, who is on a tour of the region, conveyed his "strong protest and outrage" amidst a chorus of international condemnation.

US Secretary of State Condoleezza Rice described the incident as "unfortunate," saying she has spoken to Israeli leaders about the "difficulties" caused by the shelling of the UN compound.

Louis Michel, the European commissioner for humanitarian aid, also expressed "shock and dismay" with the shelling and demanded a "full and independent" investigation into the incident.

"I am deeply shocked and dismayed to learn of this incident. Since the outbreak of the fighting I have made it very clear that all sides must respect international humanitarian law. It is unacceptable that the UN headquarters in Gaza has been struck by Israeli artillery fire," Michel said in a statement.

Meanwhile, the leaders of Germany and Britain called for an urgent action to realize a ceasefire in the Gaza Strip.

Germany and Britain will support actions designed to stop the flow of arms to Gaza and to improve the humanitarian situation there, Chancellor Angela Merkel and British Prime Minister Gordon Brown said after their meeting in Berlin.

The two leaders promised to provide technical assistance to improve security along Gaza's borders.

While continuing to carry out airstrikes and the shelling of different targets all over the Gaza Strip on Thursday, Israeli warplanes struck a 3-story building in northern Gaza, killing five, including three senior leaders of Islamic Resistance Movement (Hamas).

Hamas Interior Minister Said Siam, the head of its security apparatus, Salah Abu Shreh, and the head of its military wing, Mahmoud Watfah were killed in the airstrike.

Witnesses said that an Israeli F16 warplane fired two rockets at the building belonging to the brother of Said Siam in northern Gaza City, adding that the building was destroyed.

Palestine's UN Observer Riyad Mansour appealed to the international community to pressure Israel to end its military attacks in the Gaza Strip as the UN General Assembly (GA) kicked off an emergency session to discuss the situation.

"The international community must not shy away from using all of the diplomatic and political tools and legal instruments at its disposal to compel Israel, the occupying power, to end its military campaign ... and to end all of its illegal policies and practices ..." Mansour said.

The GA session was convened one week after the UN Security Council adopted Resolution 1860 urging an immediate ceasefire.

The European Parliament (EP), in an adopted a resolution on the situation in the Gaza Strip, called for an immediate and permanent ceasefire between Israel and Hamas, including the halt to rocket attacks by Hamas on Israel and the end of Israel's military action.

It also urged a negotiated truce that should be guaranteed by a mechanism that should be set up by the international community with coordination by the Quartet and the Arab League and could include the dispatching of a multinational presence.

The resolution encourages the diplomatic efforts so far undertaken by the international community, especially by Egypt and the European Union (EU).

The Non-Aligned Movement (NAM) demanded implementation of the UN Security Council resolution, which calls for an immediate, durable and fully respected ceasefire in Gaza, but remains unheeded by both Israel and Hamas militants.

Israel has launched unprecedented intensive airstrikes on Hamas targets since December 27. The offensive so far has killed more than 1,077 Palestinians and wounded more than 5,000 others and sparked world-wide condemnation.

(Xinhua News Agency January 16, 2009)