Israel called up reserve troops Friday and warned civilians to flee Hezbollah-controlled southern Lebanon, as it prepared for a likely ground invasion to set up a deep buffer zone.

Hezbollah militants fired at least 11 rockets at Israel’s port city of Haifa, wounding three people. Israeli warplanes pounded Lebanon’s main road link to Syria, collapsing part of Lebanon’s longest bridge. A U.N.-run observation post near the border was hit, but no one was hurt.

Ships lined up at Beirut’s port as a massive evacuation effort to pull out Americans and other foreigners picked up speed. U.S. officials said more than 8,000 of the roughly 25,000 Americans who live or work in Lebanon will be evacuated by the weekend.

After 10 days of the heaviest bombardment of Lebanon in 24 years, Israel appears to have decided that a large-scale incursion is the only way to push Hezbollah back. But mounting civilian casualties and the displacement of hundreds of thousands of Lebanese could limit the time Israel has to achieve its goals, as international tolerance for the bloodshed and destruction runs out.

An Israeli military radio station warned residents of 12 border villages in southern Lebanon to leave before 2 p.m. Friday. It was the latest in a series of recent warnings from the Al-Mashriq station, which has said Israeli forces would “act immediately” to halt Hezbollah rocket fire.

At least 335 people have been killed in Lebanon in the Israeli campaign, according to the Lebanese health minister. Thirty-four Israelis also have been killed, including 19 soldiers.

The United States — which has resisted calls for it to press its ally to halt the fighting — was sending Secretary of State Condoleezza Rice to Israel on Tuesday or Wednesday after stopping over in Arab nations, Israeli officials said. They spoke on condition of anonymity because the schedule was not yet confirmed.

The mission would be the first U.S. diplomatic effort on the ground since the Israeli onslaught against Lebanon began.

“We are all very concerned about the situation in the Middle East, and want to find a way forward that will contribute to a stable and democratic and peaceful Middle East,” Rice said Friday as she met a three-member U.N. team.

Two Apache attack helicopters collided in northern Israel near the Lebanon border, killing one air force officer and injuring three others, two seriously, Israeli officials said. Israel’s air force began an investigation.

French Foreign Minister Philippe Douste-Blazy, meanwhile, said his country was sending urgent aid to Lebanon by air and sea and he called for safe passage.

His comments came a day after U.N. Secretary-General Kofi Annan warned of a humanitarian crisis in Lebanon and called for an immediate cease-fire, even as he admitted “serious obstacles” stand in the way of even easing the violence.

“We are setting up a humanitarian air and sea port,” Douste-Blazy said in Beirut. “At the same time, we demand the establishment of humanitarian corridors.”

Top Israeli officials met Thursday night to decide how big a force to send in, according to senior military officials. They said Israel won’t stop its offensive until Hezbollah is forced behind the Litani River, 20 miles north of the border — creating a new buffer zone in a region that saw 18 years of Israeli presence since 1982.

Israel has stepped up its small forays over the border in recent days, seeking Hezbollah positions, rocket stores and bunkers. Each time it has faced tough resistance.

Israeli warplanes fired missiles that partially collapsed a 1.6-mile suspension bridge linking two steep mountain peaks, part of the Beirut-Damascus highway in central Lebanon. The bridge has been hit several times since the fighting began.

The bombing also set ablaze three buses that had just dropped off passengers in Syria, but the drivers escaped, police said.

Renewed attacks struck the ancient city of Baalbek, a major Hezbollah stronghold, and security officials said two people were killed and 19 wounded. They also attacked Hezbollah strongholds in south Beirut and elsewhere overnight.

Strikes in south Beirut killed one person, and missiles that hit a village near the border with Israel, Aita al-Shaab, killed three, officials said.

A house in the border village of Aitaroun was flattened, with 10 people believed inside, but rescuers could not reach it because of shelling, security officials said.

Air raid sirens wailed in Haifa, Israel’s third-largest city, and at least 11 rockets struck in two barrages. Three people were wounded, with 16 suffering from shock.

More rockets were fired elsewhere into northern Israel, the army said, with strikes reported in Rosh Pina, Safed and in several communities near the Sea of Galilee.

Hezbollah has fired hundreds of rockets from the Lebanese border since fighting began, forcing hundreds of thousands of Israelis to take cover in underground shelters. Eight people in Haifa were killed July 16.

A U.N. official, speaking on condition of anonymity because of the sensitivity of the situation, said an artillery shell fired by the Israeli military made “a direct hit on the U.N. position overlooking Zarit.”

An Israeli military spokesman said the position was hit by rockets fired by Hezbollah guerrillas at northern Israel. The differing accounts could not immediately be reconciled.

During an Israeli offensive against Lebanon in 1996, artillery blasted a U.N. base at Qana in southern Lebanon, killing more than 100 civilians who had taken refuge with the peacekeepers.

The U.N. mission, which has nearly 2,000 military personnel and more than 300 civilians, is to patrol the border line, known as the Blue Line, drawn by the U.N. after Israel withdrew troops from south Lebanon in 2000, ending an 18-year occupation.

Hezbollah said three of its fighters had been killed in the latest fighting with Israeli troops, bringing to six the number of guerrillas killed since Israel launched the massive military campaign against Lebanon after the militant Shiite Muslim group captured two of its soldiers on July 12.

Annan denounced Israel for “excessive use of force” and Hezbollah for holding “an entire nation hostage” with its rocket attacks and capturing the Israeli soldiers.

Neither side showed any sign of backing down.

The Israeli army issued a call-up of reserves. The exact number of troops was not disclosed, but a military official, speaking on condition of anonymity because of the sensitivity of the information, said it would be several thousand.

Hezbollah leader Sheik Hassan Nasrallah shrugged off concerns of a stepped-up Israeli onslaught, saying the captive soldiers held by his guerrillas would be freed only as part of a prisoner exchange brokered through indirect negotiations.

He spoke in an interview taped Thursday with Al-Jazeera to show he had survived an airstrike in south Beirut that Israel said targeted a Hezbollah leadership bunker. The guerrillas said the strike only hit a mosque under construction and no one was hurt.

Lebanese, meanwhile, streamed north into Beirut and other regions, crowding into schools, relatives’ homes or hotels. Taxi drivers in the south were charging up to $400 per person for rides to Beirut — more than 40 times the usual price. In remote villages of the south, cut off by strikes, residents made their way out over the mountains by foot.

The price of food, medical supplies and gasoline rose as much as 500 percent in parts of Lebanon as the bombardment cut supply routes. The World Food Program said estimates of basic food supplies ranged from one to three months.

The U.N. estimated that a half-million people have been displaced, with 130,000 fleeing to Syria and 45,000 believed to be in need of assistance.

More than 400,000 people — perhaps as many as a half-million — are believed to live south of the Litani, according to Timur Goskel, the former top U.N. adviser in the south. The river has twice been the border line for Israeli buffer zones. In 1978, Israel invaded up to the Litani to drive back Palestinian guerrillas, withdrawing from most of the south months later.

Israel invaded Lebanon again in a much bigger operation in 1982 when its forces seized parts of Beirut. It eventually carved out a buffer zone that stopped at the Litani. That zone was reduced gradually but the Israeli presence lasted for 18 years until 2000, when it withdrew its troops completely.