GAZA CITY, Gaza Strip ? Israeli tanks and troops entered southern Gaza and planes attacked three bridges and knocked out electricity to the coastal strip early today, stepping up the pressure on Palestinian militants holding captive a 19-year-old Israeli soldier.

The soldiers and tanks began taking up positions in two locations about a mile east of the Gaza town of Rafah under the cover of tank shells, according to witnesses and Palestinian security officials. Palestinians dug in behind walls and sand embankments, bracing for a major Israeli offensive.

No casualties were reported in any of the attacks, marked by the first Israeli ground offensive in Gaza since it pulled out of the territory last summer, tearing down all 21 Jewish settlements.

The Israeli strikes came amid intensive diplomatic efforts in the Arab world and by the United Nations. Secretary of State Condoleezza 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, Cpl. Gilad Shalit, could defuse the crisis, not a political agreement.

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

Early today, Israeli warplanes attacked one of the bridges for a second time, Palestinian security officials said. Before daybreak, Israeli warplanes flew low over Gaza City, causing sonic booms and breaking windows.

Overnight, Israeli planes fired at least nine missiles at Gaza’s only power station, cutting electricity to much of the Gaza Strip, Palestinian 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. Some power in Gaza City was restored by tapping into electricity supplied by Israel in northern Gaza.

Masked militants from various armed factions took up defensive positions around Gaza City, instructing drivers to turn their headlights off. Militants from three armed factions said they fired a rocket early today at the Israeli village of Nahal Oz, where Israeli forces are concentrated in a staging area. An Associated Press reporter saw tanks moving on the Israeli side of the border fence.

In the Shajaiyeh neighborhood of Gaza City, not far from the fence, armed militants took up positions across from the blaring headlights of Israeli vehicles, and Israeli attack helicopters hovered overhead. Early today, Israeli forces fired machine guns at the empty streets, witnesses said.