"The explosion happened outside an office belonging to the Afghan National Army recruitment centre," she said.

"Kabul has been largely spared this continuous low-level violence that plagues the rest of the country. The last time there was a large explosion here in the capitalwas about two months ago."

Earlier on Tuesday, a similar attack on a Nato convoy in Kabul killed at least 20 civilians, an Afghan army doctor told the AFP news agency.

"The Isaf convoy was passing ... when the suicide bomber attacked them. There might be casualties," an official told Reuters, referring to the Nato-led International Security Assistance Force.