A big explosion was heard in central Kabul early on Saturday, a Nato spokeswoman said, on the day that Afghanistan is set to go to the polls to elect a new parliament.
There was no immediate information on the cause of the explosion, which took place just before 4:00 am (local time).
"We just had a report that there was a big explosion in Kabul a couple of minutes ago, a spokeswomen for Nato's International Security Assistance Force (ISAF) told AFP.
More than 2,500 candidates are contesting 249 seats in the lower house of parliament, or Wolesi Jirga.
Polls are due to open at 7:00 am, and have come under threat from the Taliban who have been waging an insurgency in the country for almost nine years.