The attack was claimed by a Taliban spokesman, Qari Yousuf Ahmadi, who said it was against the National Directorate of Security.
A Taliban truck bomb killed at least 20 people and wounded 95 others when it exploded near a hospital in southern Afghanistan, an official said.
The blast in Zabul was just the latest in a string of deadly Taliban bombings across Afghanistan this week as violence intensifies after US President Donald Trump abruptly ended talks with the Taliban over an American withdrawal from the country.
"This morning a car bomb targeted the NDS (National Directorate of Security) in Qalat of Zabul. The regional hospital of Zabul was also located there and has been destroyed in the attack," said Rahmatullah Yarmal.
Atta Jan Haqbayan, the head of Zabul's provincial council, gave a higher death toll, saying 20 people had been killed in the attack and another 90 wounded.
The bombing was later claimed by a Taliban spokesman, Qari Yousuf Ahmadi, who said the attack was against the National Directorate of Security and had killed intelligence officers.
Watch: Afghan city of Qalat attacked by Taliban
Trump's declaration earlier this month that the talks were "dead" spurred the Taliban to declare last week that the only other option was more fighting, sparking fears of an uptick in attacks ahead of presidential elections at the end of the month.
Earlier this week, the Taliban killed nearly 50 people and wounded dozens more in two separate attacks -- one near a campaign rally for President Ashraf Ghani in the central province of Parwan, and one in Kabul.
The militants have vowed to disrupt the upcoming presidential election, scheduled for September 28, in which President Ashraf Ghani is taking on his own Chief Executive, Abdullah Abdullah, and more than a dozen other candidates.
The winner is hoping for a mandate to negotiate with the Taliban for a lasting peace in the country suffering from decades of violence.
The insurgents, however, are set on undermining the legitimacy of the process and keeping the president weak.
But even as fighting rages, the Taliban have also continued to voice their belief that the US will eventually return to the table for more negotiations.
Chief negotiator Sher Mohammad Abbas Stanikzai reiterated that stance in an interview with BBC earlier this week, stating "our doors are open for negotiations".
Trump has been eager to end America's longest war, launched nearly 18 years ago after the September 11 attacks on the US.
But he has also accused the Taliban of bad faith for launching an attack in Kabul that killed a US soldier, and others, ahead of a scheduled sit down at Camp David that was later cancelled by Trump.