Zabul shares borders with Pakistan to the southeast and Kandahar province, the birthplace of the Taliban, to the south.
The Taliban claimed responsibility for the Zabul attack in a text message from spokesman Qari Yousuf Ahmadi. He said a car bomb killed seven foreigners and wounded five others, although he later revised the toll to 13 foreigners killed and nine wounded.
The Taliban routinely exaggerates casualty figures.
The killings followed a bloody Taliban assault in the country’s west on Wednesday that killed 44 people in a courtroom in Farah province. The United Nations says civilians are being increasingly targeted.
In a statement posted online earlier on Saturday, Ahmadi said the Taliban would continue to target Afghan judges and prosecutors.
“The Islamic Emirate, from today onwards, will keep a close watch over courthouses, all its personnel and all those who try to harm Mujahideen and will deal with them the same as the judges and prosecutors of Farah.”
(Reporting by Ismail Sameem, additional reporting by Paul Eckert; Writing by Dylan Welch and Diane Bartz; Editing by Doina Chiacu and Xavier Briand)