The explosion in Jalalabad detonated early in the morning after the vehicle left the United Nations office in the city to take the workers to voter registration sites in the province, said Afghan officials on Saturday.
The attack was one of the worst against workers preparing for elections supposed to be held in September, which the Taliban and allied fighters have vowed to disrupt.
General Abd al-Malik Malikzai, a senior Afghan security official in the city, said the bomb was inside the bus. It killed two election workers and a child.
A spokesman for the UN, which is overseeing voter registration, said that according to preliminary reports, the explosion occurred as the bus was heading to Rodat district to the east of Jalalabad.
Manoel de Almeida e Silva said all the victims are Afghan nationals, adding 12 people were wounded.
He said it was too early to assess the impact on the UN-
supervised voter registration programme, but added: "It is of
great concern, not only for the process, but it is also a
matter of great sorrow - these are women who have husbands and families."
The Taliban took responsibility for the attack, the latest in a string of strikes against election workers in the lead-up to Afghanistan's first free polls after a quarter century of war.
"We did this because we warned people not to get involved
in the election process"
Abd al-Latif Hakimi,
"We did this because we warned people not to get involved in the election process," Taliban spokesman Abd al-Latif Hakimi said.
"We are also warning others not to register to vote and those who register them, because this only strengthens the foundations of the American-backed government."
Hakimi said the Taliban claimed responsibility too for the killing of two US Marines in an ambush in the eastern province of Kunar on Thursday night.
Earlier, Malikzai blamed Taliban and al-Qaida fighters, who have vowed to sabotage the vote, for the attack.
Registration of Afghan women
voters has been slow
He said the women were headed to Rodat district, east of Jalalabad, to register female voters.
Registration of female voters for the poll has lagged behind that of men, especially in conservative southern and eastern provinces where rebels are most active.
Saturday's attack came just after Afghan President Hamid Karzai appealed to NATO on Friday to make good its pledge to send more troops to protect the polls and ensure they can be held on time.
At a summit meeting in Istanbul next week, NATO is to announce that its 6400-strong peacekeeping force will take command of four or five military-civilian reconstruction teams in northern Afghanistan and deploy about 1200 troops for the polls.
But this will fall short of at least 5000 extra troops the UN and Kabul have estimated will be needed. The deployments will also be to relatively secure provinces, not to the south and east.
Leading rights group Human Rights Watch charged that NATO "foot-dragging" had contributed to worsening security. It said it should immediately send more troops to protect voter registration and disarm private armies, adding that blame for a failure of the polls would rest on Washington and NATO.