The Lebanese army advanced on Monday into a border town attacked by Islamists at the weekend in the most serious spillover of the three-year-old Syrian civil war into Lebanon, and the Beirut government said the deadly assault would not go unpunished.

With army reinforcements arriving in Arsal, Prime Minister Tammam Salam, a Sunni Muslim, said there could be no political deal with gunmen identified as members of the Nusra Front and the Islamic State, which has seized parts of Syria and Iraq.

“The only solution proposed today is the withdrawal of the militants from Arsal and its environs,” said Salam, the most senior Sunni in the Lebanese government. […]