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

Israeli warplanes also launched new air strikes 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 strikes followed bombings Wednesday that killed as many as 70 people, according to Lebanese television, making it the deadliest day since the fighting began July 12.

Russia sharply criticized Israel over its onslaught against Lebanon, now in its ninth day, sparked when Hezbollah militants captured two Israeli soldiers. The Russian Foreign Ministry said Israel's actions have gone "far beyond the boundaries of an anti-terrorist operation" and repeated calls for an immediate cease-fire.

At least 306 people have been killed in Lebanon since the Israeli campaign began, according to the security forces control room that collates casualties. In Israel, 29 people have been killed, including 14 soldiers. The U.N. has said at least a half-million people have been displaced in Lebanon.

Israel's series of small ground forays across the border have aimed to push back Hezbollah guerrillas who have continued to fire rockets into northern Israel despite more than a week of massive bombardment against them — raising the question of whether air power alone can suppress them. Guerrillas fired 25 rockets into Israel today, which caused no casualties.

But the guerrillas have been fighting back hard on the ground, wounding three Israeli soldiers today, a day after killing two. An Israeli unit sent in to ambush Hezbollah guerrillas also had a fierce gun battle today 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 one soldier. Hezbollah said in a statement that its guerrillas destroyed two tanks as they tried to enter the Lebanese border village of Maroun al-Ras, across from Avivim.

Israel has mainly limited itself to attacks 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 also broadcast warnings into south Lebanon on Wednesday telling civilians to leave the region, a possible prelude to a larger Israeli 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.

Israel dropped leaflets Wednesday night warning the population 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 its forces.

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

Lebanese Prime Minister Fuad Saniora on Wednesday appealed for a cease-fire, saying his country "has been torn to shreds." Warplanes pounded areas in the south where Hezbollah operates — but civilian residential neighborhoods bore the brunt, with dozens of houses destroyed.