A roadside bomb has killed nine civilians in eastern Afghanistan, police say, the latest attack in a surge of violence before a presidential election runoff next week.

A taxi in the eastern province of Nangarhar drove over a bomb on Friday that police said was buried beneath a road, killing all those inside.

Villagers said a tribal elder who was a passenger in the car appeared to be the target of attack.

The incident followed a similar attack a day earlier in the southern Taliban stronghold of Kandahar which killed four Afghans, including a child, the interior ministry said.

Violent campaign

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.