Israeli ground forces battled Hamas after crossing into the Gaza Strip from several directions Saturday evening, an escalation of an offensive intended to stop the militant group from launching rocket attacks against Israel.
"It will not be easy or short, but we are determined," Israeli Defense Minister Ehud Barak told reporters shortly after troops and tanks moved in.
"We are peace-seekers," Barak added. "We have restrained ourselves for a long time. But now is the time to do what needs to be done."
Around the same time, Hamas spokesman Ismail Radwan said in a television appearance that Israel was falling into a Hamas trap and vowed that Hamas would fight until defeating Israeli forces.
"Gaza will not be a picnic," a Reuters translation quoted Radwan as telling the Israelis. "Gaza will be a graveyard for you."
Both Palestinians and Israelis reported intense fighting.
Early on, Hamas claimed it had killed Israeli soldiers, though Israeli officials denied that claim.
In the first four hours of the ground invasion, there were 20 Palestinian deaths, a Palestinian medical official from the Shifa Hospital in Gaza told ABC News.
Even before troops moved in, a weeklong air assault on Hamas already had claimed the lives of more than 440 Palestinians. At least four Israelis have been killed as Hamas fired more than 400 rockets and mortars into Israel.
President George W. Bush and President-elect Barack Obama continue to be briefed on the escalating situation in Gaza. The Bush administration has so far blamed Hamas for the crisis, saying it must stop the rockets into Israel.
This evening, a statement by State Department spokesman Sean McCormack said the United States has advised Israel "to be mindful of the potential consequences to civilians" and is working toward a cease-fire that would not allow Hamas to launch rockets.
"Hamas has held the people of Gaza hostage ever since their illegal coup against the forces of President Mahmoud Abbas, the legitimate president of the Palestinian people," McCormack said. "They have used Gaza as a launching pad for rockets against Israeli cities, and have contributed deeply to a very bad daily life for the Palestinian people in Gaza and to a humanitarian situation that we have all been trying to address. Hamas has made it very difficult for the people of Gaza to have a reasonable life."
U.N. Secretary-General Ban Ki-Moon was more critical of Israel for its escalation, and he conveyed "extreme concern and disappointment" to Israeli Prime Minister Ehud Olmert, a statement from his office said.
"He is convinced and alarmed that this escalation will inevitably increase the already heavy suffering of the affected civilian populations," the statement added. "He called for an immediate end to the ground operation, and asked that Israel do all possible to ensure the protection of civilians and that humanitarian assistance is able to reach those in need."
Israeli officials said the ground offensive was the second stage of its "Operation Cast Lead" -- meant to hobble Hamas and to increase the "deterrent strength" of the Israeli Defense Forces.