KANDAHAR, Afghanistan — Insurgents, some of them wearing suicide vests, attacked one of the U.S. military’s largest and most populous bases Wednesday morning, the second ambitious attack in as many days and a sign that the Taliban movement might have launched its yearly spring offensive.

There are few installations as well-fortified as Bagram Airfield, but this did not stop insurgents from staging a predawn assault with gunfire, rockets and grenades. The fighting, which broke out at more than one location outside Bagram, killed one U.S. contractor, injured nine U.S. service members and inflicted minor damage on one of the base buildings, military officials said.

The guard force on the base battled insurgents intensely for about two hours, until about 6 a.m., but sporadic gunfire could be heard for several more hours. Ten of the attackers, including four wearing explosives, were killed in the assault, officials said.

“Though it is clear the enemy intended a spectacular event” at Bagram Airfield, “they were unable to breach the perimeter and unable to detonate their suicide vests,” U.S. Army Lt. Col. Clarence Count Jr., a military spokesman, said in a statement.

The assault on Bagram came a day after a suicide car bomber targeted a U.S. convoy in Kabul, killing five U.S. service members, a Canadian and at least 12 Afghan civilians. That was the deadliest day of the year for American troops, as two more died in separate bombings.

The Taliban claimed responsibility for both attacks. Earlier this month, the Taliban announced its own planned offensive to counter the NATO efforts that are focused on the southern city of Kandahar. The radical Islamist group called its operation “al-Fatah,” or victory.