Lebanon's militant Hezbollah group says its top military commander Mustafa Badreddine has been killed in Syria.
Hezbollah confirmed the death in a statement but did not give any further details.
Badreddine, who became the group's top commander in 2008 after his predecessor Imad Mughniyeh was assassinated, was killed by an Israeli air strike.
Badreddine was one of four people being tried in absentia for the murder of former Lebanese prime minister Rafik Hariri.
The 2005 suicide bombing that killed Hariri and 22 others was one of the Middle East's most dramatic political assassinations.
The trial began last year near The Hague and is ongoing.
A billionaire businessman, Hariri was Lebanon's most prominent politician after the 15-year civil war ended in 1990.
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The so-called Islamic State has claimed responsibility for two suicide bomb attacks in Beirut that left at least 43 people dead and more than 240 wounded.
The bombings took place in a crowded residential district in the Lebanese capital's southern suburbs - a stronghold of the Shi'ite Muslim group Hezbollah.
The attacks come at a time that Hezbollah is stepping up involvement in the Syrian civil war, sending hundreds of fighters to support President Bashar al-Assad's forces in the conflict with mostly Sunni insurgents, including IS.
The action has prompted Sunni Islamist threats against the Iran-backed group.
Thursday's blasts struck a community centre and a nearby bakery in the commercial and residential area of Borj al-Barajneh, according to security sources.
In a statement posted online by its supporters, Islamic State said its members blew up a bike loaded with explosives - with a second suicide bomber waiting for crowds to gather before detonating his own device.
At least 37 people have been killed in two suicide bombings in Beirut, Lebanon's interior minister has said.
Dozens of others were injured after explosions ripped through a busy area of a southern suburb in the capital city, which has been a stronghold of the country's Hezbollah Shi'a Islamist militant group.
Local media has said the attacks happened just outside a Shi'ite centre in the Borj al-Barajneh area.
Interior minister Nouhad Machnouk also said the attackers were on foot and wore explosive vests.
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The Middle East is on edge tonight after a missile fired by Hezbollah fighters at a military convoy killed two Israeli soldiers.
It is the Islamic Militant group's most deadly attack on Israeli forces since their five week war nearly a decade ago.
Israel has promised to do whatever it takes to defend itself.
ITV News Correspondent Dan Rivers reports:
The United States has condemned the deadly attack by Hezbollah on an Israeli military convoy that killed two Israeli soldiers, and urged both sides to "refrain from actions that could escalate the situation", according to Reuters.
"We urge all parties to refrain from any action that could escalate the situation," State Department spokeswoman Jen Psaki told a daily briefing.
A Spanish UN peacekeeper was also killed, but it is not yet known which side is responsible for his death, a UN spokesperson said.
The Spanish Prime Minister has expressed his "deepest sympathy" to the family of the Spanish UN peacekeeper who was killed during fighting near the Israeli/Lebanese border earlier today.
In a Spanish-language tweet, Mariano Rajoy Brey said: "My deepest sympathy on the death of Spanish military in Lebanon . My love and condolences to his family and companions."
Mi más profundo pesar por el fallecimiento del militar español en #Líbano. Mi afecto y condolencias a su familia y compañeros. MR
Those responsible for the deadly attack on an Israeli military convoy near southern Lebanon "will pay the full price", the Israeli prime minister Benjamin Netanyahu has said.
Mr Netanyahu was speaking ahead of consultations with Israeli security chiefs on further military response to the Hezbollah assault.