Taliban fighters killed as many as 16 men after learning they had registered for Afghanistan's U.S.-backed national elections, the deadliest attack yet in a campaign aimed at sabotaging the nation's first free vote, officials said Sunday.

The killings took place Friday in the southern province of Uruzgan, and various reports put the number of dead at 10 or 16. News of the deaths emerged a day after a bomb ripped through a bus carrying female election workers in the eastern city of Jalalabad, killing two and wounding 13. A spokesman for the Taliban took responsibility for that attack.

The violence raised security fears and added to doubts over whether Afghanistan was ready to hold elections planned for September; it also increased pressure on NATO leaders, who were to meet Monday in Turkey, to deploy more peacekeepers here.

Time is running out for the joint U.N.-Afghan electoral authority to decide on the date for the vote if the election is to be held according to schedule. According to the electoral law, the date must be announced 90 days beforehand -- meaning by July 2, if voting takes place on the last day of September.

The Uruzgan attack underscored the risks faced by Afghans attempting to exercise their democratic rights, particularly in lawless areas plagued by Taliban-led insurgents who have threatened more attacks against election workers and voters.

Rozi Khan, the Uruzgan police chief, said assailants stopped a van carrying 12 men on a road about 18 miles from the provincial capital, Tirin Kot.

When the gunmen searched the men's documents and found that they had registered to vote, they opened fire. Two men escaped and alerted police, who found the 10 bodies but had made no arrests.

Obaidullah Khan, the top political administrator of Uruzgan, confirmed the attack but said 16 people had died and only one man survived.

It was impossible to immediately account for the discrepancy.

Obaidullah Khan said six or seven attackers had launched the assault while others hid in rocks nearby.

Afghan President Hamid Karzai and the U.S. military are adamant that the election take place on schedule -- although with three days left for voter registration, only about half of those eligible have signed up nationwide.

A U.N. spokesman, Manoel de Almeida e Silva, said Sunday he expected the electoral authority to extend the deadline for registration in some areas of the country so that regional and gender imbalances in the electoral rolls can be addressed. More than 4.5 million voters have signed up so far, and only about one-third of them are women. Remote, Pashtun-dominated areas where insurgents are most active are lagging behind.

Afghan women, who as a group have been targeted by fighters aiming to sabotage the country's first free elections, register to vote in Kabul.