At least 22 people were killed and 117 people were seriously injured after Israel launched unexpected attacks on two densely populated residential buildings in central Beirut.

The Israel Defense Forces (IDF) strikes come after 27 people were killed Thursday evening when strikes hit a school that was housing locally displaced people.

Two major strikes in Beirut have already killed 22 people and injured 117, according to Lebanese health authorities, and the toll is expected to rise further.

The first major strike occurred in Bachoura, central Beirut, and was heard around 4am AEDT. People near the area heard several loud explosions and rumbling sounds.

In recent weeks, the IDF has issued immediate evacuation orders for sites it deems at high risk of harboring Hezbollah militants or materials. Strikes usually occur within an hour of warning.

However, this attack had no warning.

About an hour after the first explosion in Bachoura, rockets were heard sweeping the town before a second explosion was heard in central Beirut.

Both explosions are believed to have hit densely populated residential buildings.

Lebanese health authorities have already confirmed that 18 people were killed and around 100 injured.

Residents descended on the scene of the explosion in a desperate attempt to rescue the injured.

Large crowds are now gathering outside the American University Hospital in Beirut, where the injured were taken.

Beirut has had two relatively calm days as attacks have slowed in recent days.

However, these new strikes, which came without warning, sowed panic and chaos in the streets.

Local journalists watched as children and adults screamed in fear as Israel targeted the Hezbollah terrorist group in the city.

The Israeli military has since issued an evacuation order for southern Beirut, ordering residents to move away from a site near “Hezbollah facilities.”

“For your safety and that of your family members, you must immediately evacuate this building and adjacent buildings and move away from them for a distance of at least 500 meters,” the IDF spokesperson said.

The strikes on the two residential buildings took place as the Israeli army continued to pound Hezbollah targets across Lebanon.

Earlier in the day, an Israeli strike on a school housing displaced people in the Gaza Strip killed at least 27 people, Palestinian medical officials said.

The Israeli military said it targeted militants, but people sheltering there said the strike hit a meeting of aid workers.

Israel has continued to strike what it considers militant targets in the Palestinian enclave, although attention has focused on its war against Hezbollah in Lebanon and growing tensions with Iran.

The army launched a large-scale air and ground operation against Hamas in northern Gaza this week.

Separately, the U.N. peacekeeping force in southern Lebanon said an Israeli tank fired on its headquarters in the town of Naqoura, hitting an observation tower and wounding two peacekeepers .

The attack drew widespread condemnation and prompted the Italian Defense Ministry to summon Israel’s ambassador in protest.

The Israeli military admitted Thursday to opening fire on a U.N. base in southern Lebanon and said it had ordered peacekeepers to “remain in protected areas.”

In the past 24 hours, Lebanon’s crisis response unit said Israeli shelling and airstrikes killed 28 people and injured 113, bringing the total to 2,169 killed and 10,212 injured in the country. Lebanon since the start of the war last October.

At least four people were killed Thursday in the eastern Bekaa Valley, Lebanese health authorities said.

Hezbollah attacks have killed 28 civilians in northern Israel since the start of the war, as well as 39 Israeli soldiers, notably in northern Israel since last October and in southern Lebanon since the Israeli invasion.