Four Israeli soldiers were killed in a clash with Hezbollah guerrillas in south Lebanon, the military said Friday.

The soldiers were part of a force operating not far from the border on Thursday, looking for Hezbollah guerrillas, bases and weapons. They encountered Hezbollah forces, and a heavy exchange of fire followed, in which the four soldiers were killed and several others wounded. The military said Hezbollah also sustained losses.

Warplanes resumed strikes on targets across Lebanon on Friday as Israel warned hundreds of thousands of people to flee the south "immediately," preparing for a likely ground invasion to set up a deep buffer zone in southern Lebanon.

Israel appeared to have decided that a large-scale incursion across the border was the only way to push Hezbollah back after nine days of the heaviest bombardment of Lebanon in 24 years failed to do so.

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, 30 kilometers (20 miles) north of the border — creating a new buffer zone in a region that saw 18 years of Israeli presence since 1982.

Signs of the dramatic escalation came as U.N. chief 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.

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Annan denounced Israel for "excessive use of force" and Hezbollah for holding "an entire nation hostage" with its rocket attacks and snatching of two Israeli soldiers last week.

The United States — which has resisted calls for it to press its ally Israel to halt the fighting — was sending Secretary of State Condoleeza Rice to the region, arriving in Israel 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 nine days ago.

At least 317 people have been killed in Lebanon in the Israeli campaign, according to Lebanese security officials — at least 11 of them killed on Thursday. At least 32 Israelis have been killed, including 16 soldiers — two of them killed in Thursday's fighting. Early Friday, one air force officer died and three were injured when two Israeli helicopters collided near the Lebanese border.

Lebanese continued their exodus from the south, going on for days, streaming north to Beirut and other regions, where they crowded into schools, homes of relatives or hotels. Taxi drivers in the south where charging up to $400 per person for rides to the capital — 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 U.N. estimated that about a half-million have been displaced in Lebanon, with 130,000 fleeing to Syria and about 45,000 believed to be in need of assistance.

A World Food Program official in Lebanon, Amer Daoudi expressed concern about getting food to the displaced, saying "damage to roads and bridges has almost completely disrupted the food supply chain, hurting large numbers of the displaced."

Lebanese Prime Minister Fuad Saniora put the death toll at more than 330. He said more than 55 bridges across the country had been destroyed, and that Israeli forces had also targeted ambulances and medical convoys. "This attack is no longer against Hezbollah, it is an attack against the Lebanese and Lebanon," Saniora said.

With food and fuel supplies into the country almost impossible to replenish amid an Israeli blockade on ports and strikes on roads to Syria, prices for goods were skyrocketing — cooking gas nearly doubling to $20, some vegetables nearly quadrupling in price.

More than 600 relatives of U.N. peacekeepers and other foreigners were evacuated by ship from the southern port city of Tyre, a region south of the Litani that has seen a ferocious pounding by Israeli warplanes and gunboats for days. Many of the women and children had spent the night in the beach waiting for the ship that arrived Thursday morning and took them to Cyprus.

The escape by Americans and other foreign nationals stepped up dramatically, with ships lining up off Beirut to take thousands of families waiting at the port out of the war zone.

A group of around 40 U.S. Marines hit the ground in Beirut, helping in the evacuation of hundreds of Americans to a Navy transport vessel the USS Nashville off shore — the first U.S. military deployment in Lebanon in 22 years. More than 2,200 Americans were pulled out Thursday, twice the number a day before.

A large explosion shook Beirut shortly after daybreak Friday. Media said the strike had hit the Lebanese capital's southern suburbs, a Hezbollah stronghold. The Arab satellite TV channel Al-Jazeera said one person had been killed and another wounded, but the report could not be immediately confirmed by security officials.

Israeli aircraft also hit the town of Nabi Sheet in the eastern Bekaa valley, witnesses said. Other raids targeted a road and a bridge near the southern port city of Tyre, Hezbollah's Al-Manar TV said.

Two explosions shook south Beirut late Thursday in new Israeli strikes on the Shiite Muslim Hezbollah's stronghold in the south of the capital. During the day, Israeli strikes pounded villages and towns in the Shiite heartland of the south and the eastern Bekaa Valley.

Hezbollah, in turn, fired more than 40 rockets into northern Israel.

The clashes about 1 1/2 kilometers (1 mile) inside the Lebanese side of the border Thursday evening came when an Israeli patrol sweeping for Hezbollah bunkers was ambushed by guerillas, taking casualties. The fight rapidly expanded, with Israeli helicopters firing missiles at targets on the ground and rescue force storming in.

The Israeli military said two Israeli soldiers died in the fighting and several guerrillas were killed. Al-Manar television said three Israeli soldiers were killed but did not mention guerrilla casualties.

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

In preparation for a more powerful punch deeper into Lebanon, an Israeli military radio station that broadcasts into the south issued what it called "a strict warning" that Israeli forces would "act immediately" to halt Hezbollah rocket fire.

"It will act in word and deed inside the villages of the south against these aggressive terrorist acts. Therefore all residents of south Lebanon south of the Litani must leave their areas immediately for their own safety," the message in Arabic on the Al-Mashriq station said.

More than 300,000 people are believed to live south of the Litani — which twice has 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 June 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 from the country.

Hezbollah leader Sheik Hassan Nasrallah shrugged off concerns of a stepped up Israeli onslaught, vowing never to release two Israeli soldiers captured by his guerrillas even "if the whole universe comes (against us)." He said they would only be freed as part of a prisoner exchange brokered through indirect negotiations.

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

Israel has demanded its two captured soldiers be released and Hezbollah be disarmed before it would halt its offensive.

The European Union pledged euro10 million (US$12.5 million) Thursday in emergency aid to civilians in southern Lebanon — but its current president, Finnish Prime Minister Matti Vanhanen, admitted its demands for a ceasefire were falling on deaf ears.

The top EU foreign policy chief — Javier Solana in Cairo for talks with the Egyptian president — said countries must "use their influence to get some of the steps that have to be taken like the prisoners release, it will be very good. The sooner the better"

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