BEIRUT — Lebanese security forces arrested 17 men in two Beirut hotels on Friday on suspicion that they were plotting to assassinate a prominent Lebanese Shiite leader, a government official said, describing an attack that could inflame sectarian conflict across the Middle East.

Investigators are exploring whether the men intended to kill Nabih Berri, the speaker of Parliament, who has been a leading Shiite political figure in Lebanon for decades, the official said, speaking on the condition of anonymity under government rules. Intelligence reports identified the men as members of a newly established militant cell in Beirut that was believed to include foreigners, the official said, adding that there were suspicions that they belonged to the Islamic State in Iraq and Syria, the Sunni militant group known as ISIS.

Such a plot would be a bold and dangerous escalation by ISIS, which wields extremist and sectarian ideology and brutal tactics in its drive to erase the existing nations in the region and create a fundamentalist Islamic caliphate in their place. The group’s insurgent fighters, who already control large parts of northeastern Syria, swept across northern Iraq last week, slaughtering captured Shiite soldiers and proudly broadcasting the killings on the Internet.