The bombing came at a military roadblock near Camp Chapman in Khost province, where both Afghan and foreign troops are stationed, shortly before iftar, the meal breaking the Ramzan fast.
Mubarez Zadran, a spokesman for the provincial governor, and a senior police official confirmed the new toll, which included 27 civilians and six Afghan security personnel -- up from 18 reported killed earlier.
"Twelve children and three women are among the dead," said Zadran.
At least 12 civilians were also killed in twin roadside bomb blasts over the weekend, officials said, blaming the attacks on the Taliban.
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The insurgents launched a countrywide offensive in late April, stepping up attacks on government and foreign targets despite the nascent peace talks in what is expected to be the bloodiest fighting season in a decade.
The UN mission in the country has said almost 1,000 civilians were killed during the first four months of this year, a sharp jump from the same period last year.
In 2010, 24 Taliban militants, some wearing US uniforms, were killed when they tried to storm Camp Chapman and another nearby US base, Camp Salerno.
The base was hit again by a suicide car bomb in 2012 that killed three Afghans in a blast powerful enough to rattle windows four kilometres away.
Camp Chapman is located less than four kilometres from the city of Khost, which is near the Pakistani border, a volatile region where the Taliban, but also a multitude of armed Islamist groups, hold sway.