An Israeli security operation against the militant group Hamas "will probably not end within several days," Defense Minister Moshe Ya'alon said Tuesday.
The development, amid a barrage of rocket attacks from Gaza targeting Israeli civilians, appeared to confirm fears that the conflict would continue to escalate.
"We will not tolerate rocket fire and we are prepared to expand the operation and to exact a heavy toll on Hamas," he said.
The threat of a possible ground incursion into Gaza is clear. The Israeli military has called up several hundred reservists and is planning to call up more to strengthen positions around Gaza "with a view to possible ground maneuvers," Israeli military spokesman Lt. Col. Peter Lerner said Tuesday.
The Cabinet authorized the military to call 40,000 reserve troops. During an offensive on Gaza in November 2012, 30,000 reservists were called up.
Militants have fired more than 130 rockets at Israeli civilians since Monday night, the Israeli military said Tuesday. Summer camps were canceled and residents have repeatedly holed up in shelters as sirens wail their warnings.
Israel has "targeted approximately 150 terror sites in Gaza," the Israel Defense Forces said on Twitter.
At least 13 people have been killed in Israeli airstrikes targeting militants since Monday night, according to Palestinian sources. Among them was Mohammad Sha'aban, a leader of Hamas' militant wing. Israel confirmed that Sha'aban, "a senior Hamas terrorist," was killed.
Another militant was killed in an Israeli airstrike on an open field in Gaza.
Seven people were killed Tuesday while forming a "human shield" on the roof of a home belonging to members of Hamas' militant wing. Two were children, Palestinian medical sources said. The home was near another home belonging to the same Hamas members that Israel struck Monday night, Palestinian sources said. Earlier, the sources said 10 people had been killed on the roof.
The Israeli military, on Twitter, said Hamas "uses Palestinians as human shields when firing rockets at Israel." The tweet included a photo of what Israel said showed Hamas "caught firing a rocket" from a civilian area.
Israeli warning: Stay away from Hamas
A CNN producer was among those who received an automated phone call from the Israeli military. It warned people in Arabic to stay away from members of Hamas and other terrorist groups in Gaza and not to cooperate with them 'because they are risking your lives." The message told people not to use their homes to store arsenals for terrorist groups. "Anyone who is present in these buildings risks their lives."
The Israeli military's Operation Protective Edge against Hamas is part of an effort "to restore a state of security," Lerner told CNN on Tuesday.
Tensions in the region reached a fever pitch after three Israeli teens, including one dual U.S. citizen, on their way home from school in the West Bank were kidnapped last month. Israel blames Hamas. Their bodies were found last week.
Later in the week, a Palestinian teen was abducted and then found dead within an hour in Jerusalem. Israel has arrested suspects and says there's "strong indication" it was a revenge killing. Amid clashes in the days following, the Palestinian teen's American cousin, who was visiting, was beaten by men in Israeli security uniforms.
Israel also announced a confession in the May killing of another Israeli Jewish teen. The suspect is an Arab resident of northern Israel, and police believe the attack was fueled by Palestinian "nationalism."
Palestinian Authority President Mahmoud Abbas, who was criticized by Palestinians when he condemned the Israeli teens' kidnappings, called on Israel on Tuesday to immediately stop its strikes, warning the operation would drag the region into instability.
Abbas said a truce was needed to "spare the innocent from mass destruction."
And presidential spokesman Nabil Abu Rdainah accused Israel of carrying out an "open massacre" against children, women and the elderly, official Palestinian news agency WAFA reported. He said Israel is making a "decision to expand the war," which he said will drag "the region into a spiral of bloody violence" that "will burn everyone."
Israel says the aim of its offensive is to strike Hamas in Gaza and stop rocket fire into Israel that threatens civilians. Hamas is estimated to have 10,000 rockets of varying ranges, he said, including some that can reach as far as north as Tel Aviv and beyond.
"They have substantial armaments which can strike the soft underbelly of Israel," Lerner said.
He said the Israel Defense Forces' position had changed from focusing on de-escalation to preparing for a deterioration of the situation.
'Red lines' crossed
The conflict between the two sides has worsened in the past few days.
"The enemy has crossed the red lines and will be made to pay the price for its crimes," Mushir Al-Masri, a Hamas leadership figure and member of the Palestinian parliament, wrote on his Facebook page Monday. "The blood of our martyrs is precious ... and is fuel for the intifada and the resistance."
After that statement, the barrage of rockets from Gaza into Israel increased, with Hamas claiming responsibility.
The conflict is creating strains within the governing coalition of Israeli Prime Minister Benjamin Netanyahu.
Foreign Minister Avigdor Lieberman, head of the Yisrael Beiteinu party, said in a news conference Monday that he told Netanyahu of his intention to dissolve his party's joint faction with Netanyahu's Likud party, saying it was "not working."
Lieberman criticized Netanyahu's handling of Gaza.
Tensions are also increasing between Hamas, which controls Gaza, and the more moderate Palestinian Authority in the West Bank.
CNN's Diana Magnay reported from Jerusalem; and Ed Payne and Josh Levs reported from Atlanta. CNN's Kareem Khadder, Ben Wedeman, Jethro Mullen, and Jason Hanna contributed to this report.
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