June 28 (Bloomberg) -- Israel is ready to employ ``extreme'' tactics to free a soldier captured by Palestinians, Israeli Prime Minister Ehud Olmert said after sending armored units into the Gaza Strip today in an operation he warned may last several days.

Troops and tanks breached Gaza's southeastern border with Israel in a pre-dawn operation to free 19-year-old Corporal Gilad Shalit, the army said. Before the incursion, warplanes knocked out three bridges and cut power to parts of the territory by bombing an electricity station, it said.

``All the military activity that started overnight will continue in the coming days,'' Olmert said in a speech in Jerusalem. ``We won't hesitate to carry out extreme action to bring Gilad back to his family.''

The operation is Israel's biggest in Gaza since it withdrew from the area in September. Hopes that the pullout would reduce Palestinian attacks and help revive peace talks collapsed after Hamas won a majority in the Palestinian parliament in January and militants stepped up rocket launches from Gaza into Israel.

The incursion came hours after Hamas and Fatah, rival movements that share power in the Palestinian Authority, reached an agreement that seeks an independent state alongside Israel. While Hamas is still formally sworn to Israel's destruction, the accord implicitly recognizes the Jewish state's right to exist.

Israel's government said it is holding the Palestinian Authority responsible for Shalit's fate.

Appeal to Quartet

Palestinian Deputy Prime Minister Nasser al-Sha'er denied the government was involved in the soldier's abduction. Cabinet spokesman Ghazi Hamad called the Israeli operation ``unjustified.''

Palestinian President Mahmoud Abbas appealed to the so- called Quartet -- the U.S., the U.N., Europe, and Russia -- to intervene and put pressure on Israel to end the Gaza offensive.

``President Abbas seeks to handle the issue of the abducted Israeli soldier through peaceful means and mediation,'' said Saleh Ra'fat, a member of the Palestinian Liberation Organization's executive committee.

Chief Palestinian negotiator Saeb Erekat said that Israel ``now has control of sea, land and atmosphere.''

U.S. Reaction

In Washington, President George W. Bush's spokesman, Tony Snow, blamed Palestinian extremists for created the confrontation by kidnapping soldier and staging attacks.

While ``Israel has the right to defend itself,'' the U.S. is urging Israel to ensure that ``innocent civilians are not harmed,'' Snow said today at a White House briefing.

Olmert said the army would stay in Gaza as long as it takes to find Shalit, adding: ``We do not intend to reoccupy Gaza.''

``Our supreme mission is to bring Gilad Shalit home,'' Major General Yoav Gallant, head of southern command, told reporters in a broadcast news conference outside Gaza. If Shalit is returned, ``quiet will be met with quiet and any attack with an even harsher response,'' he said.

Most of Israel's troops were gathered in and around the shut-down Palestinian airport at Dahaniyeh, about 2 kilometers (1.2 miles) inside Gaza.

Palestinian Fire

Troops came under attack once, by Palestinians firing with light weapons and an anti-tank missile, the army spokesman said, adding there weren't any injuries. Palestinians also fired four homemade Qassam rockets that landed near Kibbutz Karmia in Israel early today, causing no damage or injuries, he said. The kibbutz is about 4 kilometers north of Gaza.

Shalit is believed by Israeli intelligence to be held in the Gaza city of Khan Yunis, about 10 kilometers from the border with Israel, Haaretz reported.

The attack on the three bridges was aimed at preventing Shalit's captors from fleeing with their hostage, the army said in an e-mailed statement. Palestinian security officials and eyewitnesses said the bridges were destroyed by F-16 jets firing missiles. No injuries were reported.

An army spokesman said that while the goal is to free Shalit, the immediate aim is to prevent the kidnappers from moving him to another location inside Gaza or to Egypt. Israel is also using its military action to pressure the Palestinian Authority, Hamas and the captors to free him, he said.

Palestinian officials said they believed Israel aims to cut the 360-square-kilometer Gaza Strip into three isolated areas.

Three Palestinian groups said to be holding the abducted Israeli soldier issued a statement two days ago saying they would release information on his condition only after Israel frees Palestinian women and those under 18 now held in its jails. At least two of the groups are affiliated with Hamas and Fatah.

To contact the reporters on this story: David Rosenberg in Jerusalem at drosenberg1@bloomberg.net; Gwen Ackerman in Jerusalem at gackerman@bloomberg.net.

Last Updated: June 28, 2006 10:50 EDT