WASHINGTON -- The Bush administration stuck to its defense of Israel's expanding offensive in the Gaza Strip on Monday and pushed for a cease-fire including a lasting halt to Hamas' rocket attacks on Israel and a reopening of border crossings that are Gaza's economic lifeline.

President George W. Bush, in his first public comments on the conflict since Israel launched a ground offensive over the weekend, said the Jewish state was justified in protecting itself against Hamas militants.

"The situation now taking place in Gaza was caused by Hamas," Bush told reporters in the Oval Office, referring to the Islamist movement that rules Gaza and is deemed by Washington to be a terrorist group.

"Instead of caring about the people of Gaza, Hamas decided to use Gaza to launch rockets to kill innocent Israelis," Bush said. "Israel's obviously decided to protect herself and her people."

While France and many other countries have pressed for an immediate cease-fire, the United States has insisted that any truce be sustainable -- and that Hamas go first in ending the exchange of fire. Hamas has launched hundreds of rockets into southern Israel, while Israel has pursued a ferocious campaign that began as a week of aerial bombing and was expanded Saturday into a ground offensive.

Bush, without explicitly endorsing Israel's ground campaign, laid blame for the conflict squarely on Hamas. He said he hopes for a cease-fire but added that it would not work unless Hamas stops its attacks.

Bush also expressed concern about the grave humanitarian crisis in Gaza, where people have lived under hard and worsening conditions and Israeli bombardment. The violence must stop, Bush said, "but not at the expense of an agreement that does not prevent the crisis from happening again."

President-elect Barack Obama said that he's been getting daily briefings on the situation in the Middle East, but offered little comment beyond noting that "delicate negotiations are taking place" and that there can't be "two voices coming out of the United States."

Obama's chief national security spokeswoman, Brooke Anderson, declined to comment on the three-point cease-fire proposal announced by the State Department, which was quickly rejected by a Hamas official as slanted in favor of Israel.

State Department spokesman Sean McCormack said Secretary of State Condoleezza Rice had 17 phone conversations over the weekend with foreign leaders in a bid for international consensus on those three points, which include a halt to Hamas' rocket firing into Israel.

The cease-fire proposal also would include a key Hamas demand: an arrangement for reopening crossing points on the border with Israel, McCormack said. The third element would address the tunnels into Gaza from Egypt through which Hamas has smuggled materials and arms.

"We're doing a lot of work on these three elements. The secretary is trying to get the international system and various actors in the international system to coalesce around those three elements," McCormack said.

McCormack said arrangements to reopen the border crossings could be worked out on the basis of a 2005 "movement and access" accord that has not been fully implemented by the Israelis and Palestinians. He said some additional equipment and technical expertise might have to be supplied as part of such an arrangement. He offered no further details.

In Damascus, Syria, a Hamas official rejected the U.S. proposal, saying it reflects the Israeli view. The deputy head of Hamas' political leadership in Syria, Moussa Abu Marzouk, told The Associated Press the U.S. plan would serve to encourage continued Israeli attacks, creating "more massacres" in Gaza.

Israel has said it intends to press its offensive until it silences the Hamas rockets, but it has been less explicit about its long-term objectives. A senior U.S. defense official said Monday that, so far, Americans see no indication inside Gaza that Israel is preparing for a long-term occupation of the territory. The defense official requested anonymity in order to discuss intelligence.

The State Department has withheld direct comment on the Israel ground thrust into Gaza, which began Saturday. Pressed for comment Monday, McCormack said, "Every sovereign state has to decide for itself how best to defend itself." He also reiterated the administration's concern about the conflict's impact on civilians.

The Gaza crisis prompted Rice to cancel a long-planned trip to China this week. While Rice has been making phone calls to allies to foster a cease-fire in Gaza, McCormack said Monday that she has no current plans to visit the Mideast as part of that effort.

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Associated Press writers Ben Feller and Pauline Jelinek contributed to this report.