•Despite all the casualties, Hezbollah's tunnel network is largely intact and American officials fear it could pose a problem for advancing Israeli forces.

•Hezbollah forces maintain dug-in positions in the hilly and easily defended terrain of southern Lebanon.

•Some officials fear that extensive attacks against Hezbollah could provoke Iran to join the fight far more openly.

The Israel Defense Forces' ground movement into Lebanon will be significant but not "major," a senior U.S. official told ABC News.

The operations will be limited to small unit commando teams, the official said, adding that the teams will have air power backup against Hezbollah fighters.

Israeli officials notified the U.S. that it planned to begin a ground incursion into Lebanon that would be limited in scope, scale and duration, and two U.S. officials tell NBC News that the operation appears to have begun.

The officials said the U.S. does not have independent confirmation, but the timing tracks with what the Israelis notified the U.S. it had planned.