WASHINGTON — American counterterrorism officials are voicing increased alarm about a Qaeda affiliate in Syria that they say is plotting attacks against the West by exploiting the chaotic security situation in the country’s northwest and the protection inadvertently afforded by Russian air defenses shielding Syrian government forces allied with Moscow.

The rise of this latest Qaeda branch in Syria, as well as the operations of other Qaeda affiliates in West Africa, Somalia, Yemen and Afghanistan, underscore the terrorist group’s enduring threat despite the death of Osama bin Laden and being largely eclipsed in recent years by the Islamic State, or ISIS, as the terrorist group of choice of global jihadis.

The new Qaeda branch, called Hurras al-Din, emerged in early 2018 after several factions broke away from a larger affiliate in Syria. It is the successor to the Khorasan Group, a small but dangerous organization of hardened senior Qaeda operatives that Ayman al-Zawahri, Al Qaeda’s leader, sent to Syria to plot attacks against the West.

The Khorasan Group was effectively wiped out by a series of American airstrikes a few years ago. But with as many as 2,000 fighters, including seasoned leaders from Jordan and Egypt, the successor Hurras al-Din group is much larger and is operating in areas where Russian air defenses largely shield them from American airstrikes and the persistent stare of American surveillance planes. Moscow dispatched military aid and advisers to Syria in late 2015 to support the beleaguered government of President Bashar al-Assad.