Hamas, meanwhile, has complained in recent months that Israel was trying to "set the rules" on the battlefield. Officials said the group stepped up attacks on Israel in recent weeks to put Israel on the defensive.
Hamas, which has ruled Gaza since seizing control of the territory five years ago, has also drawn criticism that it was going soft as it focused its efforts on building its government instead of battling Israel. Hamas, formed a quarter of a century ago as a resistance movement committed to Israel's destruction, has killed hundreds of Israelis over the years.
Hamas accused Prime Minister Benjamin Netanyahu of launching Wednesday's operation to win votes in the Jan. 22 parliamentary election. But major Israeli parties, including the dovish opposition, all lined up behind Netanyahu.
In Washington, the United States lined up behind Israel. "We support Israel's right to defend itself, and we encourage Israel to continue to take every effort to avoid civilian casualties," State Department spokesman Mark Toner said.
President Barack Obama spoke with Netanyahu and the two men agreed Hamas needs to stop its attacks on Israel to allow tensions to ease, the White House said.
Obama spoke separately to Egyptian President Mohammed Morsi, given Egypt's central role in preserving regional security, the White House said. The two men agreed on the need to de-escalate the conflict as quickly as possible.
Egypt's Foreign Minister Mohammed Kamel Amr also spoke with U.S. Secretary of State Hillary Rodham Clinton late Wednesday, asking for "immediate U.S. intervention to stop the Israeli aggression," the Foreign Ministry said in a statement.
U.N. Secretary-General Ban Ki-moon called for calm and urged both sides to respect international humanitarian law.
Federman reported from Jerusalem. Associated Press writers Karin Laub in Gaza City, and Aron Heller and Lauren E. Bohn in Jerusalem also contributed to this report.
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