GAZA CITY, Gaza Strip  -- Israeli forces have launched a "ground phase" in its effort to halt rockets being fired at Israel from Gaza, the Israeli military said today.

The incursion into Gaza "will include close coordination between [Israel's] units including infantry, armored corps, engineer corps, artillery, and intelligence combined with aerial and naval support," the Israel Defense Forces said in a statement.

The Israeli army said the offensive is meant to strike a "significant blow to Hamas' terror infrastructure."

Until today the battle has consisted of Hamas, which controls Gaza, launching rockets at Israel and Israel targeting what it calls terrorist sites in Gaza with fighter jets and naval guns. Israel twice halted its attacks and has been negotiating with Hamas for a more permanent ceasefire in Cairo, Egypt.

The IDF said today it sent troops into Gaza "Following 10 days of Hamas attacks by land, air and sea, and after repeated rejections of offers to deescalate the situation."

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The location of the "ground phase" and the number of troops involved was not immediately available, but an IDF spokesman called it "a large IDF force." The IDF also said it was throughout the Gaza Strip and that there was no time limit for the operation.

"Hamas terrorists are operating underground, and that is where the IDF will meet them. The IDF intends to impair Hamas’ capability to attack Israel,” the Israeli military said.

Israel had called up 48,000 reservists in recent weeks and the Israeli cabinet today authorized the call-up of 18,000 more reservists, bringing the total to 64,000, the IDF said. Many of the troops had been massed with tanks near Gaza's border. Israel had also warned tens of thousands of people in Gaza to evacuate their homes for their own safety.

The slugfest between Hamas and Israel has left 243 Palestinians dead and 1,850 wounded. One Israeli has been killed and several injured.

Gaza had started the day with a rare break in attacks as both sides observed a five hour "humanitarian pause" to allow Gazans to stock up on supplies. But the moment the five hour deadline arrived, a mortar round fired from Gaza hit Israeli territory, the IDF said. Israel resumed its own bombing campaign a short time later, according to the IDF.

In the hours before the Israeli incursion, powerful flares fired by Israel lit up the sky over Gaza, ABC News correspondents in Gaza reported. Foreign media in a hotel near the waterfront in Gaza were told by Israel they had to evacuate the building and tracer fire could be seen hitting sites in Gaza.

Israel last carried out a major ground offensive in Gaza in January 2009.

Earlier today, the IDF said that it thwarted an attack by 13 Gaza militants who sneaked into Israel through a tunnel. Lt. Col. Peter Lerner, an Israeli military spokesman, said the militants were identified about 820 feet inside Israel and were struck by Israeli aircraft. Lerner said the military believed at least one militant was killed in Thursday's strike. He said the remaining fighters appeared to have returned to Gaza through the tunnel.

This was the second time militants infiltrated Israel from Gaza. Israel killed four militants last week who entered from the sea.

Also today, a United Nations agency said it found 20 rockets hidden in a school it runs in Gaza.

The munitions were discovered Wednesday, the United Nations Relief and Works Agency said in a statement posted on its site today.

"In the course of the regular inspection of its premises, UNRWA discovered approximately 20 rockets hidden in a vacant school in the Gaza Strip," the agency said.

UNRWA, which supplies shelter and humanitarian aid in Gaza, condemned the storage of the weapons in its school and said, " This incident, which is the first of its kind in Gaza, endangered civilians including staff and put at risk UNRWA’s vital mission to assist and protect Palestine refugees in Gaza."

It said that they "informed the relevant parties and successfully took all necessary measures for the removal of the objects in order to preserve the safety and security of the school."

The agency didn't identify the location of the school, the types of rockets or who the "relevant parties" were.

The announcement bolstered Israeli claims that Hamas has used schools, mosques and medical facilities as shields as it fires rockets at Israel.

The battle has broken a two-year lull in hostilities between Israel and Hamas and was triggered by the slaying of three Israeli teens and revenge burning death of a Palestinian teen. Angry protests escalated to rocket launches and retaliatory air strikes.