KABUL, Afghanistan — In one of the deadliest attacks against American forces in Afghanistan this year, a Taliban suicide bomber on a motorcycle drove into a military convoy near Bagram Air Base on Monday, killing six American service members, United States officials said.

Among the dead was Detective Joseph Lemm, 45, a New York police detective serving in the Air National Guard.

The attack came as Taliban forces have continued seizing territory across Afghanistan, dragging more American troops directly into the fight despite commanders’ continued insistence that the focus of the NATO force in the country remains on training and advising. As the battle intensified this fall, the Obama administration even reversed plans to begin withdrawing troops this year, choosing to leave 9,800 American troops in Afghanistan through at least 2016.

Even away from the most intense battlegrounds, in southern and eastern Afghanistan, and with Afghan forces taking the lead in the fighting, American forces have remained at risk, with at least 15 service members killed this year before the attack on Monday.