JERUSALEM AND GAZA CITY — Israeli warplanes and helicopters bombarded military targets across the Hamas-ruled Gaza Strip on Saturday and today, retaliating for Palestinian rocket fire into Israel with one of the deadliest assaults in the history of the 60-year conflict.
As Palestinian medical officials put the death toll at 271 and said many were unarmed civilians, the scale of the bloodshed unsettled the Middle East and alarmed world leaders involved in Israeli-Palestinian peace efforts.
Hamas officials called for a new Palestinian uprising against the Jewish state and a renewed wave of suicide bombings. Despite the heavy blow to their paramilitary organization, Hamas fighters in Gaza launched at least 110 rockets into Israel, killing a man and wounding six people in the border community of Netivot.
Israeli officials warned that the surprise offensive, apparently aimed at restoring a recently lapsed cease-fire on terms more favorable to Israel, was only beginning. Hundreds of Israeli troops massed along the Gaza border early today, and civilians on both sides braced for heavy combat in the days ahead.
Prolonged fighting in Gaza would make it difficult for the Palestinian Authority's more moderate leadership in the West Bank to continue peace talks with Israel, a process guided over the last year by President Bush and a high priority for his successor, Barack Obama.
The Bush administration put the onus on the militant Islamic group. "Hamas' rocket attacks into Israel must cease if the violence is to stop," White House spokesman Gordon Johndroe said. But he added, "The United States urges Israel to avoid civilian casualties as it targets Hamas."
The number of civilian casualties in Gaza was unclear. A Palestinian Health Ministry official, Moaiya Hassanain, estimated that one-third of those killed were non-combatants. He said 388 Palestinians were wounded in more than 100 airstrikes that came in waves from midday into the early hours of today.
"Palestine has never witnessed an uglier massacre," Ismail Haniyeh, leader of the Hamas government in Gaza, declared in a statement delivered from an undisclosed location.
In a statement, the Israeli military said it had targeted Hamas security headquarters, training camps and weapons depots. "The vast majority of the casualties are terror operatives," the statement said.
Israeli officials said Hamas bore sole responsibility for any civilian casualties because it had situated many of the targets in residential neighborhoods of the territory, which is about 25 miles long and 7 miles at its widest, with a population of 1.5 million.
Official protests and street demonstrations against Israel erupted across the Middle East.
Secretary of State Condoleezza Rice, U.N. Secretary-General Ban Ki-moon and officials of the European Union, Arab League and Russia called on both sides to revive the Egyptian-arranged truce that began to break down last month and expired Dec. 19.
But Israeli Defense Minister Ehud Barak said "the operation will last as long as necessary."
"There is a time for calm and there is a time for fighting, and now is the time for fighting," Barak said at a news conference, declining to reveal whether ground troops would join the offensive.
He predicted "a trying period that will be neither easy nor short" for hundreds of thousands of Israelis living within 20 miles of the Gaza border. Israeli officials put the area on alert, warning residents to remain in "safe areas" indoors in anticipation of retaliatory fire.
Rocket fire from Gaza had become an issue in the Feb. 10 election to choose a successor to Prime Minister Ehud Olmert, who is stepping down amid corruption charges. Barak and Foreign Minister Tzipi Livni, both vying for the job, had been on the defensive as the more hawkish Benjamin Netanyahu, who leads them in recent polls, called for strong action against Hamas.
Hamas, an Islamic group backed by Iran whose charter calls for the Jewish state's destruction, won the Palestinian Authority's parliamentary elections in early 2006. Israel tightened a blockade against Gaza after Hamas seized full control of the territory after a unity government with the secular, more moderate Fatah movement collapsed in June 2007.
Israel had withdrawn its troops and settlers from the coastal strip in 2005, only to see its border communities come under frequent attacks with crude rockets fired by Hamas and smaller Palestinian groups.
Last week, after the six-month truce lapsed, Hamas stepped up its attacks, launching more than 50 rockets and mortar shells Wednesday and hitting Israeli communities slightly beyond the rockets' normal 6-mile range.