The air strike hit Dahieh, a suburb in southern Beirut.
On social media, film clips are spreading that allegedly show the aftermath of the attack, which has caused extensive damage to a residential area.
According to Lebanon's national news agency NNA, four rockets hit a multi-family house, resulting in many civilian casualties. Rescue efforts are underway among the rubble.
Sources close to Hezbollah claim, according to AFP, that Ibrahim Aqil, a high-ranking member of the Iran-backed Shia militia's military branch, was the target. Similar reports can be found in Israeli media.
The Israeli military has confirmed that a targeted attack was carried out in Beirut.
Dahieh is very densely populated and is often described as a strong Hezbollah stronghold, but it is also inhabited by many civilians.
Already tense situation
The Beirut attack was the third attack on southern Beirut since the Gaza war broke out in October last year, which has led to almost daily clashes between Hezbollah and Israel in areas near the Lebanon-Israel border.
The attack represents an escalation of the already very tense situation between Israel and Hezbollah following large-scale attacks where Hezbollah-linked communication devices exploded in Lebanon earlier this week. At least 37 people were killed and thousands injured when many of the blasts occurred in residential areas as well as in public spaces.
Earlier on Friday, rockets from Hezbollah rained down on northern Israel on a large scale. The Israeli military counts around 150 projectiles.
Has sworn revenge
The rocket attacks are described as revenge for Israeli attacks on Lebanese homes on the other side of the border.
The Israeli air force attacked hundreds of targets in Lebanon on Thursday. This happened simultaneously with Hezbollah's top leader Hassan Nasrallah holding a speech where he swore to avenge the exploding communication devices.
