But a separate Al Jazeera source said that five of those injured in the attack were children.
Ahmad said that the bomber pulled up alongside a restaurant at a bus station before the motorbike blew up.
He said the bomber had been targeting a police but detonated before reaching his target.
Other reports said the blast had been close to a taxi rank.
There was no claim of responsibility for the attack but fighters allied to the Taliban are active in southern Afghanistan and across the border in Pakistan.
The town of Spin Boldak, which is near a busy border crossing with Pakistan, has come under regular attack with strikes most often directed at the security forces although more civilians are usually killed.
The Taliban were in government for five years until 2001 when they were toppled in a US-led invasion weeks after the September 11 attacks on the US.