Lebanon's prime minister held an emergency meeting with ministers and military chiefs on Friday as his country mourned 43 people killed in a double suicide bombing claimed by Islamic State.
PM holds emergency meeting as nation mourns bomb victims
Pro-Hezbollah al-Akhbar newspaper said there was "no room for retreat", after the group warned of a "long war" against its enemies.
The blasts late on Thursday hit a residential and commercial area in a southern suburb of Beirut, a stronghold for Shi'ite Muslim group Hezbollah, in the latest spillover of violence from the war in neighbouring Syria.
"Unacceptable", newspaper L'Orient Le Jour wrote in a one-word headline across its front page on Friday.
The first attacks in more than a year on a Hezbollah stronghold inside Lebanon came at time when the group is stepping up its involvement in the Syrian civil war, now in its fifth year.
Iran-backed Hezbollah has sent troops over the border to support Syria's President Bashar al-Assad against Sunni Muslim militant groups including Islamic State.
U.N. Secretary General Ban Ki-moon urged Lebanon's security services and state institutions "not (to) allow this despicable act to destroy the relative calm that has prevailed in the country over the past year."
The White House pledged to support the country as it worked to "bring those responsible for this attack to justice".
Hezbollah's political opponents in Lebanon also condemned the attacks. Sunni politician Saad al-Hariri, who leads the Future Movement, said they were "cowardly... and unjustified" in postings on his Twitter account.
Syria's civil war is increasingly playing out as a proxy battle between regional rivals, including Iran and Saudi Arabia which support opposing sides in the conflict. The two foes also back opposing political forces in Lebanon, which suffered its own civil war from 1975 to 1990.
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