The Taliban has vowed to disrupt the runoff vote scheduled for November 7.
It warned Afghans to boycott the poll which was called after the first election in August was discredited due to widespread fraud.
A suicide attack on a UN guesthouse in the capital of Kabul on Wednesday killed five foreign staff members.
The Taliban claimed responsibility for that attack, saying it targeted the guesthouse because the UN is helping organise the runoff.
Violence in Afghanistan has reached its worst levels since the the US-led invasion ousted the Taliban in 2001.
Both military and civilian deaths have reached record levels this year.