A bomb tore through a minibus carrying Afghan women election workers in eastern Afghanistan (search) Saturday, killing at least two of them and wounding 13 in the bloodiest attack yet on preparations for the country's first post-Taliban (search) vote.

A purported spokesman for the Taliban, which has vowed to sabotage the September election, claimed responsibility for the attack.

The bus was heading from Jalalabad (search), 75 miles east of the capital, to take the election workers to register female voters in a nearby district when the blast went off Saturday morning.

U.N. spokesman Manoel de Almeida e Silva said four of those injured were in critical condition. A U.N. statement said two people were killed, correcting an earlier report that a third person -- a child traveling with his mother in the bus had died.

The attack will likely add to pressure on NATO leaders meeting in Turkey this week to make good on promises to send more peacekeepers to help secure Afghanistan ahead of the vote.

President Hamid Karzai condemned the bombing, blaming "enemies of peace and prosperity" in Afghanistan, and urged voters not to be intimidated.

The U.S. military reported that a homemade bomb exploded near the bus, but U.N. and Afghan officials said the bomb was planted inside it.

Faizan, a spokesman for provincial governor, said the bus driver, who survived the blast unhurt and fled afterward, was held for questioning.

Gen. Abdul Malik Malikzai, a senior security official, blamed Taliban and Al Qaeda insurgents for the attack, the latest targeting election workers in the lead-up to the polls.

"This is obvious that Taliban and Al Qaeda carry out bombings and explosions. They are enemies of this country," he said.

Abdul Hakim Latifi, who claims to speak for the Taliban, told The Associated Press that the bomb was detonated by remote control, and threatened more violence.

"The Taliban carried out this attack. We will not forgive any man or woman who is supporting U.S. policies. We will continue this kind of attack to make sure the elections fail," he said in a telephone call from an undisclosed location.

Despite growing security fears, and although only about half of estimated total of eligible voters are so far registered, both Karzai and the U.S. military insist the vote can go ahead. About one-third of the 4.5 million people already signed up are women.

Because of religious and cultural sensitivities in this predominantly Islamic country, voter registration is segregated between the sexes. In conservative areas of Pashtun-dominated southern and eastern Afghanistan, female electoral staff sometimes go door-to-door to encourage women to register.

Almeida e Silva said eastern Afghanistan had been successful in getting women to sign up. "The killers probably wanted to stop this momentum towards broad female participation. They will not reach their goal," he said.

Electoral authorities were restricting the movement of the women staff as the security situation was assessed, but were continuing registration of women "wherever possible."

U.N. Secretary-general Kofi Annan on Friday warned that the elections are threatened by mounting violence, and renewed a call for NATO to send more troops to Afghanistan, ahead of the two-day Istanbul summit that starts Monday.

Two British U.N. contractors were shot to death in May while trying to identify safe voter registration sites in eastern Nuristan province. Several Afghans have been injured by bombings and two heavily guarded U.N. convoys have come under fire.

Concern that instability is spreading was fueled earlier this month by the slaying of 11 Chinese construction workers and five relief workers in relatively stable northern Afghanistan.

NATO's 6,400-strong force is mainly confined to the capital, with a small German contingent in Kunduz. The alliance has pledged to expand to other northern towns in time for the vote, but member nations have been slow to provide the extra troops and equipment.

Meanwhile, the U.S. military said that two Marines killed on Thursday in eastern Kunar province died in fighting with insurgents. Spokesman Lt. Col. Tucker Mansager said that one Marine and some Afghan interpreters were wounded in the clash.

The deaths brought to at least 92 the number of American troops killed in or around Afghanistan since the start of the campaign that ousted the Taliban in late 2001.