"Up until this statement was written, there were four deaths. Two security men were martyred, in addition to a civilian employee, and an 11-year-old Syrian girl," said a ministry statement read on state television.
"The number of wounded reached 148, including 38 expatriates. Of those, 103 have left hospital and 45 remain there, three among whom are in critical condition," the statement said.
An initial toll from police and hospital sources put the number of dead at eight and wounded at 113.
"At least eight people were killed," a police official told AFP, requesting anonymity.
He said "83 people were taken for treatment to Shmaissa Hospital in Riyadh and 30 more wounded were transferred to King Faisal Specialist Hospital," also in the capital.
"An 11-year-old girl and an unidentified person succumbed to their wounds," the official said.
An official at Shmaissa Hospital said the girl was Sudanese.
Earlier, a police official said the blast killed at least six security men and wounded around 60 people.
"Six members of the security forces were killed. They include a colonel from the public security department and the department's financial director," the official told AFP.
The attack also left "around 60 people injured, including the head of the traffic department, Brigadier Fahd al-Bishr," he said.
The traffic department is one of several administrative units located in the building in the Al-Washm district. The building also houses departments charged with combating drugs and detecting explosives.
In a brief statement, an interior ministry official said the driver of a car blew up the vehicle when he was stopped from going into the headquarters of the traffic department.
"A car tried to enter the headquarters of the traffic department in Riyadh at 2:00pm (1100 GMT)," the official said, quoted by the state SPA news agency.
"When the guards there dealt with it as the situation dictates, the driver blew up the vehicle 30 meters from the entrance," the official said.
"The blast caused destruction in the surrounding area and casualties among ordinary citizens and security men," the official said without saying if they were dead or wounded.
"The sides concerned are following up the incident and a statement with the details will be issued later," he added.