At least one rocket has struck the northern Israeli port city of Haifa, hours after a threat by the militant Lebanese group Hezbollah.
Hezbollah denied firing any rockets. There were no immediate reports of injuries or damage.
Haifa, Israel's third largest city, is more than 30km (18 miles) from the Lebanese border and was thought to be out of Hezbollah's range.
Israel has been carrying out ground, sea and air raids on Lebanon.
It imposed the blockade following the capture of two of its soldiers by Hezbollah militants on Wednesday.
Israeli warships have blocked Lebanese ports, and Beirut international airport was closed after Israeli bombing.
About 50 Lebanese people, mostly civilians, have been killed in the raids.
Hezbollah had said it would attack Haifa if Israeli planes bombed Beirut.
It has fired dozens of rockets into Israel in the past two days, but none have so far gone further than 20km (12 miles) inside the country.
At least one Israeli has died in the attacks and dozens have been injured.
The Israeli ambassador in Washington, Danny Ayalon, described the Haifa incident as a "major escalation" of the crisis, which began with the soldiers' capture.
