Kabul, Afghanistan -- At least 13 U.S. troops were killed in central Kabul on Saturday when a suicide bomber struck a vehicle in a NATO military convoy, a U.S. military official said.
NATO's International Security Assistance Force confirmed 13 deaths within its force, but did not specify their nationalities.
The attack caused a "number" of NATO and local Afghan casualties, ISAF said in a statement. Four Afghans, including two students, were also killed, said Hashmat Stanikzai, spokesman for Kabul's police chief.
A Taliban spokesman confirmed Saturday's attack in a text message, saying it killed "16 foreign soldiers, one civilian" and injured many others.
Taliban casualty counts are often inflated; there was no other reliable indication 16 foreigners were killed.
Stanikzai said the vehicle used in the attack appeared to be a red Toyota Corolla packed with a significant amount of explosives.
It was unclear how many people were wounded, said Sediq Sediqqi, a spokesman for the Interior Ministry.
The deaths were the largest single-day U.S. loss in Afghanistan since the August crash in Afghanistan's Wardak province that killed 38 people, including 17 Navy SEALS. That aircraft was brought down by an insurgent rocket-propelled grenade.
The attack was one of two targeting NATO-led forces that day.
A gunman wearing an Afghan army uniform turned his weapon on coalition forces during training, killing two, said Master Sgt. Christopher DeWitt, a spokesman for ISAF. The shooter was killed in the incident in southern Afghanistan.
The coalition did not provide any other details about the shooting, and did not disclose the nationalities of those killed.
In another suicide attack in northeastern Afghanistan, a woman in a burqa detonated herself near the nation's intelligence agency.
She tried to enter the National Directorate of Security and was shot at, but she still managed to detonate herself, said Sabour Alayar, deputy police chief of Kunar province.
Two officers and two civilians were wounded, he said, adding that the female suicide bomber was about 25 years old.
Alayar said they had intelligence of a suicide bomber looking for a target, and their security forces were on alert.
The August helicopter attack made for the the deadliest month for U.S. forces in Afghanistan in the 10 years since allied forces began their campaign there. Seventy-one American troops died in August, six more than in July 2010, which previously had been the worst month for U.S. casualties.
Fifteen U.S. soldiers and three civilian contractors were killed in April 2005 when a coalition helicopter traveling in severe weather crashed near Ghazni. Sixteen Americans -- eight soldiers and eight sailors -- were killed when their MH-47 helicopter was downed by a rocket-propelled grenade near Kunar province in June 2005.
In May 2006, a U.S. helicopter crashed near Asadabad in Kunar province, killing all 10 U.S. soldiers aboard.
Three Drug Enforcement Administration special agents and seven U.S. troops were killed in western Afghanistan in October 2009 when they returned from a raid on a compound believed to be harboring insurgents tied to drug trafficking.
The U.S.-led war in Afghanistan marked its 10th year earlier this month having passed two major milestones: The Taliban has been forced out of power and Osama bin Laden is dead.
But Afghanistan has been hit by a wave of high-profile attacks in recent months that have jeopardized the peace negotiations.
September's turban bomb assassination of former Afghan President Burhanuddin Rabbani, revered by many as a father of the Mujahedeen movement that ousted the Soviets in the 1980s, appears to have dealt the biggest blow to the peace process.
Rabbani was the chairman of President Hamid Karzai's High Council for Peace, which has been trying for a year to foster dialogue with the Taliban -- a strategy that Karzai publicly abandoned following Rabbani's killing.
Nearly 2,800 troops from the United States and its partners have died during the 10 years of war, according to a CNN count.