Israel is continuing to subject Lebanon to strikes by land, sea and air, following the capture of two Israeli soldiers by Hezbollah militants.
Hezbollah has fired dozens of rockets into northern Israel
More than 50 people, mostly civilians, have been killed in the attacks.
Israeli jets have struck southern Beirut, a Hezbollah stronghold, and the main Beirut-Damascus road.
Hezbollah has hit Israeli towns with rocket attacks and Israeli sources said the group had fired on the northern port city of Haifa.
Hezbollah had said it would attack Haifa if Israeli planes bombed Beirut but denied firing any rockets at Israel's third largest city.
However, it has killed at least two Israelis and injured dozens in rocket attacks into Israel in the past two days.
The Israeli ambassador in Washington, Danny Ayalon, described the Haifa incident as a "major escalation" of the crisis.
Israel later bombed targets in southern Beirut, where Hezbollah leader Sheik Hassan Nasrallah has offices, Lebanese security sources said. A power plant was also reportedly hit.
And Lebanon's road to Damascus - the main arterial route - was closed by Israeli jet attacks in the central mountains region, Lebanese officials said.
On Thursday, Israeli forces twice struck the international airport in Beirut.
Mr Ayalon said the international community should make it clear to Iran and Syria - which Israel says form an "axis of terror" with Hezbollah and Palestinian militants Hamas - that they were "playing with fire".
Calls for calm
According to Syrian television, Iranian President Mahmoud Ahmadinejad telephoned Syrian President Bashar al-Assad on Thursday night to say there would be a "fierce response" to any Israeli attack on Syria.
"If the Zionist regime commits another stupid move and attacks Syria, this will be considered like attacking the whole Islamic world," Mr Ahmadinejad was quoted as saying.
ISRAEL IN LEBANON
March 1978: Israel invades to stop Palestinian attacks
1982: Full-scale invasion; Israel occupies Beirut; pro-Israel militias massacre Palestinian refugees
May 1983: Israel pulls back, but keeps "security zone"
February 1992: Israeli air strike kills Hezbollah leader
1996: Israel launches "Grapes of Wrath" raids on Hezbollah; 100 civilians die under Israeli shelling of UN base at Qana
May 2000: Israel withdraws troops from Lebanon
January 2004: Prisoners-bodies swap agreed between Hezbollah and Israel
Meanwhile the UN Security Council has arranged an emergency meeting for Friday at Lebanon's request.
International calls for calm are growing, with Russia, France and the EU saying Israel's response to the capture of two soldiers was disproportionate.
US Secretary of State Condoleezza Rice urged Israel to exercise restraint but also demanded that Syria put pressure on Hezbollah to stop attacks on Israel.
Lebanese ministers have called for a ceasefire with Israel, saying that all means should be used to end "open aggression" against their country.
Israeli government spokesman Mark Regev said Israel was responding to "an unprovoked act of aggression" by Lebanon.
The offensive follows a day of heavy fighting on Wednesday in which the Israelis suffered their worst losses on the border for several years.
Eight soldiers were killed and two were injured, in addition to the two captured in a Hezbollah ambush.
Hezbollah has said the captured soldiers will not be returned without a release deal for Palestinian, Lebanese and other Arab prisoners held in Israeli jails.
Israel is also continuing a separate offensive in the Gaza Strip. An Israeli soldier was captured in a cross-border raid by Palestinian militants last month.
ISRAEL BLOCKADES LEBANON
Two rockets hit Israeli city of Haifa on Thursday evening
Israel targets Lebanon by land, air and sea: enforcing naval blockade, bombing Beirut airport and shelling Lebanese towns
Jets bomb Lebanese army air base at Rayak and Baalbek TV transmitter in Bekaa Valley
Shelling from both sides is heaviest over Lebanon's southern border
Hezbollah targets Kiryat Shmona, Nahariya and Safed in Israel