Five people have died after an explosion hit a vehicle being used by election staff in south-eastern Afghanistan, a UN spokesman says.
Security was a concern in the run-up to the poll
At least one election official died in the attack in Paktia province, Manoel de Almeida e Silva said.
The incident follows landmark elections on 9 October, which Afghanistan's Taleban vowed but failed to disrupt.
Interim leader Hamid Karzai has a 45-point lead after the first million votes were counted in the election.
The car hit by the explosion was owned by the joint UN-Afghan election commission.
The attack took place on Monday near the town of Yahya Khel but it is not known whether election work was deliberately targeted.
A police official quoted by the Associated Press, Mohammed Rahim Alikhel, identified the election worker as a local doctor who had helped organise the vote.
Fraud is favouring Karzai, says rival Qanuni
The other victims were the doctor's nephew, his driver and two other local men, the official said.
Mr Almeida e Silva said the area was known for having many mines and homemade bombs.
Roadside bombs, rockets and other devices have been commonly used by the Taleban for attacks before and after the presidential election.
About 13% of the total ballots cast have now been counted, drawn from 29 of Afghanistan's 34 provinces.
Officials acknowledge there have been problems with the poll, especially with ink used to mark people's hands to stop repeat votes.
On Monday Mr Karzai's main rival, Yunus Qanuni, accused organisers of "robbing the people's vote".
Mr Qanuni claims to have evidence of organised fraud in favour of Mr Karzai and that the UN-Afghan electoral commission was ignoring his complaints.
"If His Excellency Mr Karzai, my old friend, succeeds in a fair and transparent election, I will congratulate him and co-operate with him," Mr Qanuni told reporters.
"But if the result is fraudulent, the legitimacy of this election will be in question."
A three-member panel established by the United Nations and the UN-Afghan Joint Electoral Management Body (JEMB) is investigating charges of irregularities.