Two Israeli soldiers were killed in a Hezbollah strike on an army convoy near the Lebanese border. Another seven soldiers were wounded in the attack.
Israel's military responded to the anti-tank missile strike on the convoy with aerial and ground attacks on Hezbollah positions in Lebanon.
A Spanish United Nations peacekeeper was killed as Israel conducted air strikes after the attack.
Smoke rises from shells fired from Israel over farmland in southern LebanonKaramallah Daher/ReutersSmoke rises from shells fired from Israel over al-Wazzani in southern LebanonKaramallah Daher/ReutersSpanish soldiers of the United Nations Interim Force in Lebanon (UNIFIL) drive an armoured emergency vehicle after picking up the body of a 36-year-old UN peacekeeper from Spain who was killed when the Israeli military shelled border areas following a Hezbollah attack that left two Israeli soldiers deadMahmoud Zayyat/AFPA member of the United Nations Disengagement Observer Force (UNDOF) looks through binoculars at Mount Bental, an observation post in the Israeli occupied Golan HeightsBaz Ratner/Reuters
A retired Israeli army officer, Major-General Israel Ziv, said he believed the assault was an attempt by Hezbollah to draw Israel more deeply into the war in Syria, where Hezbollah is fighting alongside forces loyal to President Bashar al-Assad.
"Israel understands that we need to contain things," he said. "Israel needs to protect its interests but not take any unnecessary steps that may pull us into the conflict in Syria."
Prime Minister Benjamin Netanyahu, who has made security his top priority ahead of parliamentary elections on 17 March, said Israel was "prepared to act powerfully on all fronts," adding: "Security comes before everything else."
His office accused Iran of being behind what was described as a "criminal terror attack". Iran is a major funder of Hezbollah.
Hezbollah members and religious figures mourn over the coffin of Lebanon's Hezbollah commander Mohamad Issa, known as Abu Issa, during his funeral in Arab-Salim, south Lebanon on January 20Ali Hashisho/ReutersAhmad, the son of Lebanon's Hezbollah commander Mohamad Issa, carries his father's rifle during his funeral in Arab-Salim, on January 20Ali Hashisho/ReutersIranian mourners carry the coffin of Brigadier General Mohammad Ali Allahdadi, a commander of the Revolutionary Guards who was killed in an Israeli air strike, during his funeral procession in Tehran on January 21Atta Kenare/AFP
Tensions in the region, where the frontiers of Israel, Lebanon and Syria meet and militant groups opposed to Syrian President Assad are active, have been bubbling for months but have boiled over in the past 10 days.
The frontier has largely been quiet since 2006, when Hezbollah and Israel fought a 34-day war in which 120 people in Israel and more than 500 in Lebanon were killed.
In Beirut, celebratory gunfire rang out after the attack, while residents in the southern suburbs of the city, where Hezbollah is strong, packed their bags and prepared to evacuate neighbourhoods that were heavily bombed by Israel in 2006.
Supporters wave Hezbollah flags in Beirut's southern suburbs, after the group fired a missile at Israeli military vehiclesKhalil Hassan/ReutersHezbollah supporters release fireworks in front of a billboard of Lebanon's Hezbollah leader Sayyed Hasan Nasrallah in BeirutHasan Shaaban/Reuters