GAZA CITY, Gaza Strip – Israeli troops entered southern Gaza and planes attacked two bridges and a power station, knocking out electricity in most of the coastal strip early yesterday and stepping up the pressure on Palestinian Arab terrorists holding captive a 19-year-old Israeli soldier.

Israeli troops began taking up positions in two locations east of the Gaza town of Rafah under the cover of tank shells, according to witnesses and Palestinian Arab security officials. Palestinian Arabs dug in behind mounds of dirt, bracing for a major Israeli offensive.

The Israeli strikes came amid intensive diplomatic efforts in the Arab world and by the United Nations. Secretary of State Rice urged Israel to “give diplomacy a chance.”

Trying to defuse building tensions, negotiators from the ruling Hamas movement said Tuesday they had accepted a document implicitly recognizing Israel. But two Syrian-based Hamas leaders denied a final deal had been reached. Israel said only freedom for the captive soldier, Corporal Gilad Shalit, could defuse the crisis, not a political agreement.

Meanwhile, a Palestinian Arab terrorist group threatened to kill an abducted Jewish settler if Israel doesn’t stop its raid on the Gaza Strip.

The Popular Resistance Committees, which has claimed responsibility for abducting both the settler and an Israeli army corporal, said in a statement that “we are running out of patience.” Israeli authorities have not confirmed the settler’s abduction.

The Israeli military said in a statement that the object of the attacks on the bridges late yesterday and early today was “to impair the ability of the terrorists to transfer the kidnapped soldier.” Knocking down the bridges would cut Gaza in two, Palestinian Arab security officials said.

Early yesterday, Israeli planes fired at least nine missiles at Gaza’s only power station, cutting electricity to much of the Gaza Strip, Palestinian Arab security officials said. The station’s three functioning turbines and a gasoline reservoir were engulfed in enormous flames that firefighters were unable to control.

The attack raised the specter of a humanitarian crisis in Gaza, as water pumps in the strip are powered by electricity.

Israeli military officials said Prime Minister Olmert approved a “limited operation” for southern Gaza, aimed at “terrorist infrastructure.” The officials spoke on condition of anonymity because they were not authorized to talk to reporters.

Palestinian Arab security forces said Israeli tanks were on the move near the Israeli village of Nahal Oz, a main Israeli staging area just outside Gaza, but that they had not yet entered the territory. An Associated Press reporter saw tanks moving on the Israeli side of the border fence.

Some Israeli troops crossed the border into southern Gaza, near the site of Sunday’s terrorist attack in which Corporal Shalit was abducted. It was not immediately clear how many soldiers entered Gaza, though the army confirmed its soldiers were crossing the border. A number of Israeli soldiers had been in Gaza since Sunday’s assault.

In the Shajaiyeh neighborhood of Gaza City, not far from the fence, armed terrorists took up positions across from the blaring headlights of Israeli vehicles, and Israeli attack helicopters hovered overhead. The roar of Israeli fighter planes reverberated throughout Gaza City.

The terrorists told residents to leave the area. They piled gasoline-soaked tires in the streets. Earlier, bulldozers blocked some of the main roads with piles of sand and dirt to try to slow down Israeli tanks.

There were no reports of casualties in the Israeli strikes. Palestinian Arab TV showed pictures of the first bridge hit, with fallen concrete blocks, twisted metal, and protruding water pipes. Children walked in the wreckage.

Corporal Shalit’s abduction Sunday by Hamas’s military wing and two other Hamas-linked groups has threatened to turn already devastated relations between Israel and the Hamas-led government into an all-out war. Hamas took over the Palestinian Authority after winning parliamentary elections in January, and has been under international pressure to renounce violence and recognize Israel.

The White House press secretary, Tony Snow, said he had only seen press accounts of the Hamas-Fatah accord, but reiterated that Hamas had to meet three conditions before a crippling aid boycott could be lifted.

“Once again, we can all recite from memory now: Recognize Israel’s right to exist, renounce terror, and abide by all past agreements. Those are the preconditions,” Mr. Snow said in Washington.

Complicating matters was a new claim by the Hamas-linked Popular Resistance Committees, one of the three groups that carried out Sunday’s assault, that it had also kidnapped the Jewish settler in the West Bank. The group later issued the death threat.

A police spokesman, Micky Rosenfeld, said the report was being taken “very seriously,” and military officials said there was “rising fears” the claim was true.

The fate of the abducted soldier has riveted Israelis, with Corporal Shalit’s face plastered on newspapers and callers to talk shows praying for his safety.

In an interview Tuesday with the Associated Press, Noam Shalit begged the captors of his wounded son’s to provide medical care and asked “to hear his voice and to see his face.”

Mr. Olmert rejected the kidnappers’ demands to free Palestinian Arab prisoners and instead approved plans for a military push into Gaza. About 3,000 soldiers, along with tanks and armored vehicles, massed along Israel’s border with the territory, and commanders said they were awaiting orders to move in.

Hamas’s Web site said there were “back channel” negotiations with Israel over a prisoner release.

Israeli military officials said a negotiating team has been activated, but declined to release further information.

[Bloomberg News reported yesterday that the Hamas and Fatah movements, which share control of the Palestinian Authority, agreed yesterday to end a dispute over how to deal with Israel by backing a document that implicitly recognizes the Jewish state’s right to exist.

The movements, along with other factions, agreed to accept the terms of the “prisoners’ document,” which calls for a Palestinian Arab state in the West Bank, Gaza Strip, and East Jerusalem, said Sallah Zeidan, an official from the Democratic Front for the Liberation of Palestine, one of the groups in the talks. The areas were captured by Israel in the 1967 Six-Day War.

“The Palestinian factions participating in the national dialogue have agreed to the document of national accordance that was drafted by leaders of Palestinian prisoners in Israeli jails,” he said in a telephone interview.]