The U.S. military said Wednesday six American troops were killed in Afghanistan, as militants killed six election workers amid growing fears on the eve of the presidential election that insurgents would mar the vote.

Two troops were killed in gunfire in the south on Wednesday, the U.S. military said, while a third was killed in an unspecified hostile attack. The U.S. also said a roadside bomb Tuesday in the south killed two troops, while another died of noncombat-related injuries. No other details were released.

The deaths bring to at least 32 the number of American troops killed in the country this month, a record pace. Forty-four U.S. troops died in Afghanistan last month, the deadliest month of the eight-year war.

Attacks in the countryside killed six election workers, officials said Wednesday, one day before Afghanistan decides whether President Hamid Karzai deserves a second five-year term. In Kabul, three Taliban militants took over a bank, and gunfire and small explosions reverberated throughout the capital. Police stormed the bank and killed the three militants.

The drumbeat of attacks would appear to signal the intent of Taliban insurgents and their militant allies to disrupt Thursday's vote.

Karzai faces some three dozen presidential candidates at the polls, including his former foreign minister and top challenger, Abdullah Abdullah. Islamist insurgents have threatened violence against those who take part in the election -- a crucial step in President Barack Obama's campaign to turn around the deteriorating war.

Afghanistan's electoral commission said all but one of the country's 364 districts had received voting materials. Polls open at 7 a.m. Thursday.

In a region generally considered safe, four election workers were killed Tuesday when their vehicle struck a roadside bomb about 20 miles outside the capital of northeastern Badakhshan province. Officials said the four were delivering materials to a polling station.

Another two election workers were killed in Shorabak district of Kandahar province on Tuesday when their vehicle hit a roadside bomb, said Abdul Wasai Alakozai, the chief electoral officer for southern Afghanistan.

A remote-controlled roadside bomb exploded early Wednesday near a vehicle taking voting supplies to a poll in the Chaparhar district of the eastern province of Nangarhar, said Ahmad Zia Abdulzai, the governor's spokesman. The driver was slightly wounded, but the voting materials were not damaged, he said. Security forces arrested the man who detonated the bomb, he said.

The Interior Ministry says about a third of Afghanistan is at high-risk of militant attack, and that no polling stations will open in eight Afghan districts under control of militants.

The three armed men took over a branch of the Pashtani bank early Wednesday in a section of Kabul's old city still in ruins from the country's 1990s civil war. Police surrounded the building, exchanging gunfire with the attackers.

Abdul Ghafar Sayedzada, head of Kabul's criminal investigations unit, said police eventually stormed the building and killed three "terrorists." Few civilians were in the area because government ministries and businesses were closed Wednesday in observance of Afghanistan's independence from British rule.

Taliban spokesman Zabiullah Mujahid said 20 armed suicide attackers wearing explosive vests had entered Kabul and that five of them battled police. The claim could not be confirmed, but the Taliban in recent months have unleashed several attacks involving teams of insurgents assaulting government or high-profile sites.

The latest attacks were an ominous sign that the Taliban and their militant allies are determined to disrupt Thursday's election.

U.S. Secretary of State Hillary Rodham Clinton said Tuesday that the rise in insurgent violence in Afghanistan reflected a deliberate campaign to intimidate voters. A shopkeeper near Wednesday's gunfire attack in Kabul, Abdul Jalal, said that if violence persisted into Thursday, he and his wife would not vote.

"Tomorrow we plan to go the polling center," said Jalal. "But if it was like today, we will not vote. Elections are a good thing for Afghanistan, but security is more important."

Attacks nationwide have increased in recent days from a daily average of about 32 to 48, said Brig. Gen. E. Tremblay, the spokesman for the NATO-led force. Even with the increase, Tremblay said that insurgents do not have the ability to widely disrupt voting at the country's 6,500 or so polling sites.

"When you're looking purely at statistics ... they're not going to be able to attack even 1 percent of the entire polling sites in this country," he said on Tuesday.

U.N. Secretery-General Ban Ki-moon encouraged all Afghans to vote and said that by participating in the election Afghans will help "bring fresh vigor to the country's political life, and ultimately reaffirm their commitment to contribute to the peace and prosperity of their nation."

The next president will face challenges on several fronts: the rising Taliban insurgency, internal political divisions, ethnic tensions, unemployment, the country's drug trade and corruption.

Karzai is favored to win, but if he does not get more than 50 percent of Thursday's vote he and the second-place finisher will face off in an October run-off. Polls show Abdullah in second place with around 25 percent support and Karzai's support around 45 percent.

Preliminary official results of the presidential election should be announced sometime Saturday evening.

Fearing that violence may dampen turnout, the Foreign Ministry issued a statement Tuesday demanding that news organizations to avoid "broadcasting any incidence of violence" between 6 a.m. and 8 p.m. on election day "to ensure the wide participation of the Afghan people."

In other violence, a roadside bomb killed a district government leader and a tribal elder early Wednesday in the Registan district of Kandahar, said Ghulam Ali Wahadat, a police commander in southern Afghanistan.

Another roadside bomb in Tirin Kot, in Uruzgan province, killed three policemen, said Ali Jan, a provincial police official.