The strikes came after militants from Lebanon fired nearly three dozen rockets at Israel on Thursday.
Additionally, rockets were fired from Gaza toward southern Israel early Friday morning, following the Israeli attacks there.
Clashes have been escalating this week, after Israeli police raids at the Al-Aqsa Mosque compound in Jerusalem.
With the Islamic holy month of Ramadan and the weeklong Jewish holiday of Passover coinciding again this year, the cross-border attacks have come at a delicate time.
The site of the mosque is known to Muslims as "Haram al-Sharif" and is the third holiest site in Islam. It is known to Jews as the Temple Mount, where the Wailing Wall, or Western Wall, is — the most sacred site in Judaism.
Two years ago, claims over an Israeli police operation at Al-Aqsa led to 11 days of open conflict between Israel and Palestinian militants in Gaza.
Israel says it was targeting Hamas installations in Lebanon
The Israeli military on Friday said the strikes targeted installations belonging to the militant group Hamas in the south of Lebanon. Local media reported blasts in the Lebanese port city of Tyre.
The US, EU, Japan and several other nations have classified Hamas as a terrorist organization.
Following a meeting with his Security Cabinet late on Thursday, Israeli Prime Minister Benjamin Netanyahu said, "Israel's response, tonight and beyond, will extract a heavy price from our enemies."
Lebanon's Prime Minister Najib Mikati said his country "categorically rejects any military escalation."
In response to the strikes, Hamas said they "strongly" condemned "aggression against Lebanon in the vicinity of Tyre at dawn today."
UN urges de-escalation
Meanwhile, the United Nations Interim Force in Lebanon (UNIFIL) urged calm on Friday. The UN peacekeepers — deployed in southern Lebanon to provide a buffer between the two countries — called on "all parties to cease all actions" on both sides of the border.
The latest flare-up of rocket fire between Lebanese territory and Israel is one of the largest escalations of violence along the border since Israeli forces and paramilitary forces allied with the Lebanon-based organization, Hezbollah, fought a 34-day war in 2006.
British Foreign Secretary James Cleverly also called on "all parties" to lower tensions while also condemning the rocket attacks on Israel.
"The UK condemns the indiscriminate rocket attacks from southern Lebanon and Gaza and recognizes Israel's right to self-defense. Now is the time for all parties across the region to de-escalate tensions," Cleverly said in a statement issued by his office.
Britain's top diplomat also called for "places of worship to be respected" and condemned "Israeli police violence at the Al-Aqsa Mosque," and said that security operations needed to be "proportionate and in accordance with international law."
2 Israelis killed in West Bank shooting
Israeli authorities said on Friday that two Israeli women were killed and another injured in a shooting attack on their car in the Israeli-occupied West Bank.
The Israeli Defense Forces (IDF) said that the incident took place at the Hamra Junction, which is close to the Jewish settlement of the same name.
The IDF said that soldiers were "blocking routes adjacent to the scene of the shooting attack" and are currently pursuing the suspected assailants.
There has been no immediate claim of responsibility or information on the suspects.
kb, jsi/rs (AP, AFP, dpa)