Israel said on Thursday its naval ships were in Lebanon's territorial waters to enforce a blockade after Hezbollah guerrillas captured two soldiers in a border clash.
Israeli planes earlier attacked Beirut's international airport in another step to sealing access.
Israel's Army Radio said other airports in Lebanon could be targeted too.
"Since this morning Israeli naval vessels have enforced a full naval closure on Lebanon, because Lebanon's ports are used to transfer both terrorists and weapons to the terror organisations operating in Lebanon," a military spokesman said.
The spokesman declined comment on whether Israel was also enforcing a closure of Lebanon's airspace.
Israeli media said the aim was to put pressure on the Lebanese government to act against Hezbollah, which seized the soldiers and killed seven others in an attack Wednesday. The government in Beirut has said it is not responsible.
Israeli air raids on Lebanon killed 27 civilians on Thursday.
"We will take all necessary measures to bring an end to the rogue behaviour of a state that does not assume responsibility and put an end to the activities of a major partner in their own government," cabinet minister Isaac Herzog said.