BEIRUT, Lebanon (AP) – Israeli troops met fierce resistance from Hezbollah guerrillas Thursday as they crossed into Lebanon to seek tunnels and weapons for a second straight day, and Israel hinted at a full-scale invasion.

Israel warned residents to “immediately” flee a nearly 20-mile swath of south Lebanon along the border. Its warplanes also launched new airstrikes on Beirut’s southern suburbs, a Hezbollah stronghold, shortly after daybreak, followed by strikes in the guerrillas’ heartland in the south and eastern Bekaa Valley.

The planes also bombed large parts of the south, from which Hezbollah guerrillas fired more rockets into Israel. On Wednesday, Israeli bombings killed as many as 70 people, according to Lebanese television, making it the deadliest day since the fighting began July 12.

A large fight between Israeli forces and Hezbollah guerrillas broke out Thursday evening on the Lebanese side of the border, the Israeli army said, adding that its troops suffered casualties but did not elaborate. Hezbollah’s Al-Manar television said three Israeli soldiers were killed and 10 wounded in fighting.

The forces crossed the border as part of ongoing operations to push back Hezbollah guerrillas who have continued firing rockets into northern Israel despite more than a week of massive bombardment.

U.N. Secretary-General Kofi Annan told the Security Council that “hostilities must stop” between Israel and Hezbollah. He also condemned Israel’s “excessive use of force” in Lebanon.

“There are serious obstacles to reaching a cease-fire or even to diminishing the violence quickly,” Annan said.

Secretary of State Condoleeza Rice will travel to the region as early as next week, said State Department spokesman Sean McCormack. She will discuss the Mideast on Thursday evening in New York with Annan and the European Union foreign policy chief.

The fighting had triggered a humanitarian crisis, he added. 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.

Russia sharply criticized Israel’s onslaught, now in its ninth day, sparked when Hezbollah militants captured two Israeli soldiers. Moscow said Israel’s actions have gone “far beyond the boundaries of an anti-terrorist operation.”

At least 306 people have been killed in Lebanon since Israel’s campaign began, according to Lebanese officials. At least 29 Israelis have been killed, including 14 soldiers.

About 40 U.S. Marines landed in Beirut to help Americans onto the USS Nashville, which will carry 1,200 evacuees bound for Cyprus in the second mass U.S. exodus from Lebanon. It was the first U.S. military deployment in Lebanon in 22 years.

Thousands of Europeans also fled on ships _ continuing one of the largest evacuation operations since World War II. An estimated 13,000 foreign nationals have been evacuated.

More than 600 relatives of U.N. peacekeepers and other foreigners were evacuated by ship from the southern port of Tyre, a region that has been pounded for days by Israeli warplanes and gunboats.

Hezbollah guerrillas fired 25 rockets into Israel on Thursday. Although they caused no casualties, the continued rocket barrage raised the question of whether Israeli air power alone can suppress them.

The guerrillas have been fighting back hard on the ground, wounding three Israeli soldiers. An Israeli unit sent in to ambush Hezbollah guerrillas also had a fierce gunbattle with a cell of militants.

In another clash, just across the border from the Israeli town of Avivim, guerrillas fired a missile at an Israeli tank, seriously wounding a soldier. Hezbollah said its guerrillas destroyed two tanks trying to enter the Lebanese border village of Maroun al-Ras, across from Avivim.

In the Gaza Strip, where Israel has been fighting for three weeks after one of its soldiers was captured, Israeli forces killed three people and wounded six Thursday. Nine people _ eight of them militants _ were killed a day earlier.

Israel has mainly limited itself to attacks in Lebanon from the air and sea, reluctant to send in ground troops on terrain dominated by Hezbollah.

But an Israeli army spokesman refused to rule out the possibility of a full-scale invasion. Israel broadcast warnings Wednesday into south Lebanon, telling civilians south of the Litani River to “leave their areas immediately for their own safety” _ a possible prelude to a larger ground operation.

“There is a possibility _ all our options are open. At the moment, it’s a very limited, specific incursion but all options remain open,” Capt. Jacob Dallal, an Israeli army spokesman, told The Associated Press on Wednesday.

Leaflets dropped Wednesday night warned that any trucks traveling in Lebanese towns south of the Litani River would be suspected of carrying weapons and rockets and could be targeted by Israeli forces.

A Hezbollah official said it was “fully ready” for an Israeli ground offensive, dismissing Israeli claims to have destroyed half the guerrillas’ arsenal of missiles. Mahmoud Koumati, deputy leader of Hezbollah’s political bureau, told the Lebanese Broadcasting Corp. the group has enough missiles to fight Israel for “long months.”

The Lebanese government is under international pressure to deploy troops in the south to rein in Hezbollah _ but even before the fighting, many considered it too weak to do so without deeply fracturing the country.

On Wednesday, Lebanese Prime Minister Fuad Saniora appealed for a cease-fire, saying Lebanon “has been torn to shreds.”

Dallal said Israel had hit “1,000 targets in the last eight days _ 20 percent were missile-launching sites and the rest were control and command centers, missiles and so forth.”

Brig. Gen. Ido Nehushtan insisted the Israeli army never targets civilians but has no way of knowing if they are in an area it is striking. “Civilians might be in the area because Hezbollah is operating from civilian territory,” he said.

He said that Hezbollah has fired more than 1,100 rockets at civilian areas in Israel since the fighting began and that 12 percent _ or about 750,000 people _ of Israel’s population lives in areas that can be targeted by the guerrillas.

The Israeli military said aircraft dropped 23 tons of explosives on what it believed was a bunker for senior Hezbollah leaders in the Bourj al-Barajneh neighborhood of Beirut between 8 p.m. and 9 p.m. Wednesday.

Hezbollah said none of its members was hurt and denied a leadership bunker was in the area, saying a mosque under construction was hit. It has a headquarters compound in Bourj al-Barajneh that is off limits to Lebanese police and army, so security officials could not confirm the strike.

Israel’s U.N. Ambassador Dan Gillerman told CNN his country would not comment about the attack until it is sure of all the facts. But he added, “I can assure you that we know exactly what we hit. … This was no religious site. This was indeed the headquarters of the Hezbollah leadership.”

On Thursday, Israeli jets struck houses believed used by Hezbollah officials in the town of Hermel in the western Bekaa Valley, wounding at least three.

Israeli warplanes also destroyed a five-story residential and commercial building that reportedly once held a Hezbollah office in the Bekaa Valley city of Baalbek, a Hezbollah stronghold, witnesses said. There was no immediate word on casualties.

Two civilians were killed late Wednesday in strikes on bridges in Lebanon’s far north, near Tripoli, the National News Agency said.

Israeli jets also raided a detention center in the southern town of Khiam Thursday, witnesses and local TV said. The notorious Khiam prison, formerly run by Israel’s Lebanese militia allies during its occupation, was destroyed, they said.

International pressure mounted on Israel and the United States to agree to a cease-fire.

The destruction and rising death toll deepened a rift between the U.S. and Europe. The Bush administration is giving Israel a tacit green light to take the time it needs to neutralize Hezbollah, but the Europeans fear mounting civilian casualties will play into the hands of militants and weaken Lebanon’s democratically elected government.

U.N. High Commissioner for Human Rights Louise Arbour criticized the rising toll, saying the shelling was invariably killing innocent civilians.

“International law demands accountability,” she said in Geneva. “The scale of the killings in the region, and their predictability, could engage the personal criminal responsibility of those involved, particularly those in a position of command and control.”