Afghan soldiers and police and Nato-led forces cordoned off the site.
The blast gouged a crater into the tarmac and damaged the bus.
Suicide attacks are a key feature of an insurgency started by the Taliban movement after it was driven from government in late 2001 by a US-led coalition. The US military said there were 139 last year.
Meanwhile, on Monday, the defence ministry reported that its troops had killed two "enemy" in the southern Zabul province and seized weapons and motorbikes "used for terrorist activities".
Commanders of the nearly 40,000 foreign troops trying to help the government gain control of the country expect more suicide attacks this year as they are easier than engaging troops in conventional battle.
There have already been several, including one in the eastern city of Khost on January 23 that killed eight Afghan labourers queueing to enter a US base for work and two Afghan security guards.
The European Union said on Monday it would contribute $775m in aid over the next four years, largely to bolster the judiciary in order to fight corruption.