AVA- Fourteen more were wounded in the blast which happened in Hammam market, Sargardan Square in the provincial capital city, also Khost.
Mubariz Mohammad Zadran, a spokesman for provincial governor, said that the blast was triggered by explosives.
He said that the victims were all civilians.
There was no immediate claim of responsibility for the bombing.
The incident comes as President Ashraf Ghani has travelled to Turkmenistan to attend the inauguration ceremony of 5th Asian Indoor and Martial Arts Games in Ashgabat.
From there, he will travel to the United States to attend the U.N. General Assembly debate on Tuesday.
On Thursday, he will meet with his U.S. counterpart Donald Trump. During the meeting the sides will discuss mutual cooperation, security, peace and counter-terrorism, Ghani’s deputy spokesman Dawa Khan Meenapal told reporters in Kabul on Sunday.