A Lebanese girl peers from a window of her family's damaged house today as she looks at the rubble of a bridge. An Israeli missile strike destroyed the bridge, located south of the port town of Sidon, Lebanon.

BEIRUT, Lebanon — A U.N.-run observation post near the border took a direct hit today during fighting between Israel and Hezbollah militants. Israel resumed airstrikes on Lebanon and prepared for a possible ground invasion, warning people in the south to flee.

The Israeli army said Hezbollah rockets hit the U.N. post near Zarit, just inside Israel, but a U.N. officer said it was an artillery shell fired by the Israeli Defense Force. The facility was severely damaged, but nobody was injured as the Ghanian troops manning the post were inside bomb shelters at the time of the strike, the U.N. official said.

Israeli warplanes also pounded Lebanon's main road link to Syria with missiles and set passenger buses on fire, police said, adding that part of Lebanon's longest bridge collapsed.

Two Apache attack helicopters collided in northern Israel near the Lebanon border early today, killing one air force officer and injuring three others, two seriously, Israeli officials said. Al-Jazeera reported that four soldiers were killed in the crash, but did not give a source. The commander of Israel's air force appointed an inquiry team to determine the cause.

French Foreign Minister Philippe Douste-Blazy, meanwhile, said his country was dispatching 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 told reporters during a visit to Beirut. "At the same time we demand the establishment of humanitarian corridors."

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

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 from the guerrillas.

Airstrikes left three passenger buses in flames in the Bekaa Valley near the Syrian border, on the road linking Beirut and Damascus, but police said nobody was hurt. The buses had just dropped off foreign passengers in Syria.

Israeli warplanes also fired four missiles that caused the collapse of part of a 1.6 mile-long 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.

Also today, heavy black smoke billowed as Israeli warplanes renewed attacks on the ancient city of Baalbek — a major Hezbollah stronghold. Warplanes also attacked Hezbollah strongholds in south Beirut and elsewhere overnight.

The Arab satellite TV channel Al-Jazeera said one person had been killed in south Beirut and another wounded, but the report could not be immediately confirmed by security officials.

The U.N. official, speaking on condition of anonymity because of the sensitivity of the situation, said an artillery shell fired by the Israeli Defense Force "impacted a direct hit on the U.N. position overlooking Zarit."

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

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