Israel has hit the Lebanese capital, Beirut, for only the second time since the start of a ceasefire last month.
The Israel Defense Forces (IDF) said the attack at about 14:00 (11:00 GMT) was carried out in a "targeted manner", but gave no details. Israeli media cited unnamed sources as saying the target of the strike had been the head of an Iranian militia.
Israel had spared Beirut at the request of US President Donald Trump and the attack came after waves of Israeli strikes aimed at Hezbollah infrastructure in southern Lebanon, according to the IDF.
Both Israel and Hezbollah - the powerful Shia group supported by Iran - have accused each other of repeated violations of a ceasefire.
Thick smoke was seen billowing across residential buildings in Dahieh, the densely populated Shia suburb that serves as Hezbollah's stronghold in the capital, after the latest Israeli attack.
Residents could be heard calling out to neighbours and relatives to check on their safety as rescue crews rushed in.
According to Israeli media, the target of the strike was Ali al-Husni, head of the missile force in the Imam Hossein Division - an Iranian militia allied to Hezbollah.
The strikes came after Israeli Prime Minister Benjamin Netanyahu announced an expansion of a ground operation following Hezbollah drone attacks on troops occupying part of southern Lebanon and on civilians in northern Israel.
On Wednesday, the IDF urged residents to move north of the Zahrani River, about 40km (25 miles) from the Israeli border, saying it would act "with extreme force".
At least 11 people were killed in two sets of Israeli strikes which hit Tyre and an area to the city's east early on Thursday, according to the Lebanese health ministry.
Social media videos from Tyre, one of Lebanon's biggest cities, showed streets lit orange by flames, smoke-filled roads, and at least one vehicle engulfed in fire.
By daylight, a massive fireball was filmed erupting near a cluster of high-rise residential buildings, sending a mushroom-shaped column of smoke rising above the city skyline.
Stunned residents looked on as debris spread through surrounding streets.
A Hezbollah member in Tyre told the BBC rescue and recovery crews had been forced to stop their work because conditions remain "too dangerous" and workers received calls from the Israeli military warning them to evacuate the area.
Wednesday's evacuation order was the largest since the ceasefire took effect, covering about 300 towns and villages - about 14% of Lebanese territory. Many residents, including those already displaced from other parts of southern Lebanon, have nowhere obvious to go.
The streets of Saida were oddly busy on Thursday - the marina area filled with beach goers eating lunch, and not a tent for the displaced in sight.
But with shelters exceeding capacity, humanitarian workers and city officials told those displaced to keep going north. There is no more room here.
Saida is a coastal city north of the Zahrani River, but south of Beirut. It has not been hit as hard as other cities like Tyre or Nabatieh.
It was not named in the latest set of evacuation orders, so Hanaa Jamaa, 46, was shocked when she was woken up in the middle of the night to hear an apartment she owned had been hit.
She had been renting it out as a source of income.
A missile struck the building at approximately 02:40, appearing to hit the roof before tearing downward through the structure.
Four people were killed in the building - two displaced people and two residents from Saida.
The man renting the apartment from Hanaa had been living there for three years.
She said he was a civilian. "We aren't with Hezbollah and we aren't with Israel," she said through tears. "We just want peace."
Israeli officials have said Hezbollah's attacks are violating the temporary ceasefire deal between the Israeli and Lebanese governments, which has been extended twice since it came into force last month.
Lebanese officials have pointed to the Israeli strikes themselves as violations.
The escalation threatens to derail talks aimed at ending the war between the US and Israel on one side and Iran on the other. Iran insists that any deal must also cover Lebanon. Israel says it reserves the right to continue to fight the threat from Hezbollah.
Lebanon was drawn into the war on 2 March, when Hezbollah launched rockets into Israel in retaliation for an Israeli strike that killed Iran's supreme leader. Israel responded with an air campaign across Lebanon and a ground invasion.
At least 3,224 people have been killed in Lebanon since the start of the war, according to the country's health ministry - its figures do not distinguish between combatants and civilians.
Israel says 23 of its soldiers and four Israeli civilians have been killed over the same period on both sides of the border.
Additional reporting by Angie Mrad.
