EDGARTOWN — Declaring that the beheading of an American journalist was a terrorist attack on the United States, the Obama administration said Friday that it was weighing how to confront Islamic State militants in Syria, in what would be a major escalation of U.S. efforts to defeat the extremists.

President Barack Obama, who sought to build a legacy as a leader who ends wars rather than starts them, until now had resisted direct U.S. intervention in the more than 3-year-old Syrian civil war.