Locals told Anadolu Agency that the attacker apparently targeted the Afghan security forces' convoy in Khost City, the provincial capital of Khost.
Najib Danish, deputy spokesman for the Ministry of Interior said in a tweet that the attack -- which Taliban immediately claimed as part of their ongoing annual offensives -- had left 18 people dead, and six others injured including two children.
It is not yet clear how many civilians and security forces personnel have been killed in the attack.
Mohammadeen Mohammadi, head of the Public Health Department in the province, told Anadolu Agency at least one injured civilian was in critical state.
Afghan President Ashraf Ghani condemned the deadly attack, which he said invited the wrath of the people towards the Taliban.
"Terrorist groups that have no regard for religious values and blessed days, have once again committed a crime against humanity on the directives of their masters," Ghani said in a statement.
"Criminals must know that by killing and targeting innocent civilians, they cannot not reach their goals, but receive more hatred from the people," he added.
The High Peace Council, tasked with reaching a peace deal with the Taliban, termed the attack as "painful and very disturbing".
Mohammad Ismael Qasimyar, adviser at the HPC, told Anadolu Agency that such attacks were a setback to efforts for peace.
"We urge the Taliban to shun violence, and at least cease fire in the month of Ramadan," he said.
The attack was also widely condemned by the general public on social media, with one Tweeter asking "On the basis of what religion are you [Taliban] waging these atrocities?".
Meanwhile, the Ministry of Defense announced in its daily security roundup that 53 militants had been killed in the past 24 hours in ongoing counter-terrorism operations in 18 out of 34 provinces.
The first day of Ramadan is a public holiday in Afghanistan. In clear defiance to the call of the United Nations Assistance Mission in Afghanistan (UNAMA) for cease-fire in this holy month, the Taliban announced Friday they would in fact intensify their attacks during Ramadan.