SIDON, Lebanon, July 12 — The militant Shiite Muslim group Hezbollah captured two Israeli soldiers along the Israel-Lebanon border Wednesday morning, and Israeli officials said seven more soldiers were killed after tanks and troops moved into Lebanon in response to the attack.

Israeli Prime Minister Ehud Olmert called the abduction of the soldiers an “act of war” and said Hezbollah would pay a “heavy price,” the Associated Press reported. The kidnappings follow the June 25 capture by Palestinian gunmen of an Israeli soldier in the Gaza Strip.

Israel will hold the government of Lebanon “fully responsible” for Hezbollah’s actions, Defense Minister Amir Peretz said in a statement. He said the government “must act immediately and seriously to locate” the soldiers, “to prevent any harm done to them and to return them to Israel.”

The Israeli forces entered Lebanon soon after the 9 a.m. abduction. At least three soldiers were killed in a blast involving one of the tanks, Israeli officials said. Preliminary reports said four other soldiers also had been killed.

In southern Lebanon, fighter jets bombed five bridges in quick succession, effectively cutting off that region from the rest of the country, civil defense officials said. At least two Lebanese civilians were killed in one of the bridge strikes, and a power plant was badly damaged.

Air strikes hit the cities of Marjuyun and Kfar Shouba. Warships were shelling Lebanon, Israeli news agencies reported, and witnesses said Katyusha rockets were being fired from the western part of the border into Israel, close to the Mediterranean Sea.

Scores of suddenly stranded Lebanese wandered back roads looking for a way home — their faces grim and worried, their belongings stuffed into plastic bags. Sirens wailed in the background.

“We’re scared, we’re scared, from the moment of the attack until now, we’re just scared,” said Um Fatimah, whose cousin was one of the people killed in the air strike on the bridge. “It only gets more difficult for us.”

After warplanes passed over Beirut, anti-aircraft fire thundered through the capital for about a half hour. The Israeli government urged residents of northern border towns to seek cover in underground bomb shelters.

The attacks and counterattacks are sure to escalate tensions along a border that often serves as a battlefield between Hezbollah militias and the Israeli army. But the sense of unease and fear was mixed with resignation from people who have known peace only rarely. Many also said they were elated at the capture of the soldiers by Hezbollah, which for years has effectively controlled this region.

“Look, we’re used to it — 25 years, 26 years it’s been like this,” Hassan Qaryani, a 21-year-old butcher from Burj Rahal, said of the air strikes. The kidnapping, he said, was “like a crown on my head… as soon as I heard the news I was overjoyed. It was like Italy winning the World Cup.”

In the southern suburbs of Beirut, people handed out candy in the streets and set off fireworks. Fireworks also were set off on the airport road, snarling traffic.

Tensions in Israel were already extremely high, because of ongoing efforts to free Gilad Shalit, a soldier captured 17 days ago by Palestinian militants in Gaza who snuck through a cross-border tunnel and attacked his Army post.

Early this morning, before the situation exploded in Lebanon, Israel expanded its military offensive in Gaza, dropping a quarter-ton bomb on a house in Gaza City. Israeli officials said they targeted the house based on information that senior Palestinian gunmen were meeting there to plan a terrorist attack.

The huge explosion destroyed the house of Hamas activist Nabil Abu Salmiyeh, a lecturer at Gaza City’s Islamic University, killing him, his wife and seven of their nine children, the Associated Press reported. Rescue workers said four people were still missing, and 37 were wounded, according to the wire service. The dead children ranged in age from 4 to 18.

Israeli officials said leaders of the Hamas military wing were meeting in Abu Salmiyeh’s home and accused the militants of using civilians as a shield. Officials initially said they believed Mohammed Deif, leader of the armed wing of Hamas, had been wounded in the attack. But Hamas officials said in a mobile phone text message that Deif was unharmed, wire services reported.

Israel blames Deif, a master bomb maker, for many suicide attacks against Israeli targets. The bombing raid was the army’s fourth attempt to kill him; he lost an eye in a 2002 missile strike.

The exact circumstances of this morning’s abductions in Lebanon remained unclear. Reports in Lebanon suggested the two soldiers were seized after they crossed the border to investigate rocket attacks. But Israeli officials said they believe Hezbollah militants may have entered Israel to capture the soldiers.

Hezbollah Secretary General Hasan Nasrallah in the past has threatened to abduct Israeli soldiers as a way of winning the release of three Lebanese prisoners still held in Israel.

Lt. Gen. Dan Halutz, chief of staff for the Israel Defense Forces, spent much of Wednesday meeting with his top staff and coordinating the military activity on the northern border. Olmert called an emergency cabinet meeting for tonight.

“The government of Lebanon, which allows Hezbollah to freely operate against Israel from within its jurisdiction, must bear the responsibility for the ramifications of this action,” Peretz’s statement said. “The State of Israel will take any measure it sees fit and the IDF will be instructed accordingly.”

Wilson reported from Israel, Wilgoren reported from Washington. Staff writer Fred Barbash contributed to this report from Washington.