A blast outside an army recruitment centre in the capital Kabul has killed at least six people, sources say.

Abdul Ghafar Sayedzada, the head of criminal investigations for the police in Kabul, told the Reuters news agency that Tuesday's explosion took place near the national parliament "but we have no more details".

Al Jazeera's Hoda Abdel-Hamid, reporting from Kabul, said the explosion was "very loud" and could have been caused by "a suicide bomb or a car bomb".

"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.