President Hamid Karzai's powerful cousin has been killed in a suicide-bomb attack at his home near the southern province of Kandahar, officials says.
Hashmat Karzai was an election-campaign manager in Kandahar for Ashraf Ghani, one of the two presidential candidates involved in a dispute over fraud that threatens to push the country into a new spell of instability.
Hashmat Karzai, was killed by a man with explosives hidden inside his turban when visitors arrived to celebrate Eid on Tuesday, the holiday marking the end of the Muslim holy month of Ramadan.
"A suicide bomber disguised as a guest came to Hashmat Karzai's house to greet him," Dawa Khan Minapal, the Kandahar provincial governor's spokesman, told AFP news agency.
"After he hugged Hashmat, he blew up his explosives and killed him."
This is not the first time that President Karzai's family members have been targeted. His brother, Ahmad Wali Karzai, was killed in his home in the city of Kandahar by an unknown attacker seven years ago.
Ghani, a former finance minister, and his rival, former foreign minister Abdullah Abdullah, say the vote was marred by fraud, and the UN has sent a team of observers to oversee an audit of the ballot.
The new president had been due to be sworn in next month.
Major delays could complicate plans for an agreement to keep about 10,000 US troops in the country after most troops leave at the end of 2014.
Ghani, a former World Bank official, condemned the killing of his adviser.
"[We] condemn this act, of the enemies of AFG, in the strongest terms," Ghani said on Twitter.
No one else was killed and security agents were investigating, the governor's office told Reuters.
The two candidates agreed to an audit of all the vote's cast in a second round run-off after John Kerry, the US secretary of state, brokered a deal, but the process is moving slowly, bogged down by frequent disagreements.