While the Islamic State group is getting the most attention now, another band of extremists in Syria — a mix of hardened jihadis from Afghanistan, Yemen, Syria and Europe — poses a more direct and imminent threat to the United States, working with Yemeni bomb-makers to target U.S. aviation, American officials say. At the center is a cell known as the Khorasan group, a cadre of veteran al-Qaida fighters from Afghanistan and Pakistan who traveled to Syria to link up with the al-Qaida affiliate there, the Nusra Front.