The latest death was reported as Afghan officials announced an 80 percent increase in the number of major fraud allegations submitted after last week's disputed presidential election - a sign of the deep challenges facing the U.S. and its allies in shoring up a legitimate Afghan government capable of withstanding the Taliban insurgency, corruption and drug trafficking.
A brief statement by the NATO command gave few details of the blast and did not say precisely where it occurred. U.S. military spokeswoman Capt. Elizabeth Mathias said the service member who died was American.
That brought to 45 the number of U.S. service members killed this month in the Afghan war - one more than the previous monthly record, set in July.
American casualties have been rising steadily following President Barack Obama's decision to send 21,000 additional troops to Afghanistan to combat a resurgent Taliban and train Afghan security forces to assume a greater role in battling the insurgents.
Obama's decision was part of a strategic shift in the U.S. war against international Islamic extremism - moving resources from Iraq, which had been center stage since the 2003 U.S.-led invasion but where violence has declined sharply from levels of two years ago.
A record 62,000 U.S. troops are now in the country, with 4,000 more due before year's end. That compares with about 130,000 in Iraq, most due to leave next year.
Since the fresh troops began arriving in Afghanistan last spring, U.S. deaths have climbed steadily - from 12 in May to more than 40 for the past two months as American forces have taken the fight to the Taliban in areas of the country which have long been under insurgent control.
At least 732 U.S. service members have died in the Afghan war since the U.S.-led invasion of late 2001. Nearly 60 percent of those deaths occurred since the Taliban insurgency began to rebound in 2007.
The latest spike in U.S. deaths has raised doubts among the United States and its allies about the course of the war, which was launched by the Bush administration after the Taliban government refused to hand over Osama bin Laden for his role in the Sept. 11, 2001 terror attacks in the United States.
A recent Washington Post-ABC News poll found that just over 50 percent of the American respondents said the war in Afghanistan was not worth fighting. Anti-war sentiment is also growing in Britain following a spike in deaths among British forces in Afghanistan.
The debate over the war is likely to accelerate when the new top commander in Afghanistan, Gen. Stanley McChrystal, submits an assessment of the conflict by the end of this month.
McChrystal, who commanded special operations troops in Iraq, is expected to give a bleak assessment of the war, pointing to deficiencies in the Afghan government and recommending vastly expanding the size of Afghanistan's own security forces.
Those weaknesses in the Afghan government have come into sharp focus since the flawed Aug. 20 presidential election, which produced allegations of widespread fraud - most leveled by opponents of President Hamid Karzai.
Final results are not expected for weeks, but preliminary figures released this week show Karzai leading the 36-candidate field with 44.8 percent of the vote, followed by ex-Foreign Minister Abdullah Abdullah with 35.1 percent. A runoff must be held if no candidate wins more than 50 percent. Abdullah has accused Karzai of rigging the election, a charge the incumbent denies.
On Friday, the U.N.-backed Electoral Complaints Commission said the number of major fraud complaints which could "materially effect" the outcome had soared to 270. On Wednesday, the commission said it had received 150 major complaints, which could delay announcement of the final results.
The lengthy election process has added to strains in U.S.-Afghan relations, which had already cooled since the Obama administration took office.
On Friday, two officials said Karzai angrily accused the U.S. of pushing for a runoff vote during a heated meeting with the special envoy to the region.
According to officials familiar with the encounter, the verbal exchange occurred the day after the Aug. 20 vote during a meeting in Kabul between Karzai and U.S. special envoy Richard Holbrooke. The officials were briefed about the meeting and spoke on condition of anonymity because of the sensitivity of the information.
Karzai assured Holbrooke he would accept the election results but bristled when Holbrooke asked if he would also agree to a runoff.
An angry Karzai accused the U.S. of urging a second round before all votes had been counted. Karzai said he would accept the election commission's tabulation as long as it reflected the facts. He did not elaborate, according to the officials.
The U.S. Embassy confirmed the Aug. 21 meeting and said the two discussed the election but would not go into details.
"There was no shouting and no one stormed out," said Caitlin Hayden, an embassy spokeswoman. She noted Holbrooke and Karzai met again a few days later. Karzai spokesman Humayun Hamidzada also confirmed the meeting but gave no further details.
Karzai enjoyed close ties with the Bush administration, which helped propel him to power after the collapse of the Taliban government in the U.S.-led invasion.
Since the Obama administration took office, U.S. officials have accused Karzai of weak leadership as well as tolerating corruption and a flourishing drug trade.
The New York Times reported this week that the Obama administration is alarmed at the prospect that Karzai's running mate, Mohammad Qasim Fahim, may be linked to the drug trade.
Quoting an unidentified administration official, the newspaper said if Fahim becomes vice president, the U.S. would likely consider imposing sanctions such as refusing him a U.S. visa or going after his personal finances.
A U.S. official in Washington confirmed the essence of the report, saying there were "a number of individuals" whom the U.S. would not like to see in a future Afghan government. He spoke on condition of anonymity because of the sensitive subject matter.
Associated Press writers Heidi Vogt and Nahal Toosi in Kabul and Matthew Lee in Washington and AP researcher Monika Mathur in New York contributed to this report.