A blast ripped through a mosque where Afghans were registering to vote on Wednesday, killing at least two people, and the Medecins Sans Frontieres aid agency said it was leaving the country after 24 years because of security fears.

The attack in Ghazni province was the worst on poll preparations since three women election workers were killed by a bomb in Jalalabad on June 26, a day after 16 Afghans found holding voter registration cards were shot dead in the south.

U.N. spokesman David Singh said two people were killed in the Ghazni blast, one of them a member of the Afghan election coordinating body, and two election officials were seriously wounded. The U.S. military said six people had been killed.

Authorities have blamed all three attacks on remnants of the ousted Taliban regime and Islamic militant allies, who have vowed to disrupt a presidential vote in October and parliamentary elections planned for April.

More than 900 people have been killed in a wave of violence during the past year that has targeted foreign and local troops, aid workers and people involved in preparing for the country's first free, direct elections.

MSF, or Doctors Without Borders, the Nobel prize-winning aid organization, said it was leaving Afghanistan because of fears for the safety of staff after five workers, including three foreigners, were killed in an attack on a remote road in the northwest in June.

The group issued a stinging rebuke of U.S. forces in Afghanistan, saying they had used aid work to help them win a "hearts and minds" campaign and garner support from Afghans skeptical of their intentions.

"MSF denounces this attempt to co-opt humanitarian aid, to use humanitarian aid to win hearts and minds," MSF secretary general Marine Buissonniere told a briefing, adding that in doing so it had endangered the lives of aid workers.