Last Updated 9:36 a.m. ET
KABUL, Afghanistan - A U.S. official said all 13 NATO service members killed in a suicide bombing in the Afghan capital were American troops.
The car bomb struck an American convoy in Kabul, hitting an armored bus carrying U.S. soldiers which then burst into flames.
The official confirmed the nationalities shortly after NATO issued a statement saying that 13 of its forces were killed in Saturday's blast. The official spoke on condition of anonymity because of the sensitivity of the issue.
The attack also killed four Afghan civilians, including a policeman and schoolchildren
CBS News correspondent Mandy Clark, reporting from Kabul, said helicopters landed near the scene and air-lifted a number of casualties, according to eyewitnesses.
"It was a huge blast," one eyewitness said. "I went closer to the convoy and saw several Americans on fire."
The Taliban claimed responsibility for the attack. They said the bomber had more than 1,500 pounds of explosives in his car.
The Taliban insists they will continue to target foreign forces until they all pull out of Afghanistan, said Clark.
Violence across the country is at its worse since the start of the war ten years ago, according to the United Nations, despite more than 130,000 foreign troops on the ground.
The attack was the deadliest of three separate incidents Saturday that targeted either the U.S.-led coalition or Afghan government offices in the country.
In the south, an area traditionally viewed as the Taliban's stronghold, NATO said a man in an Afghan military uniform turned his weapon on coalition and Afghan forces, killing two. The shooter was killed, NATO said in the statement that provided no other details.
NATO and Afghan forces sealed off the area as rescuers rushed about the attack site, according to an Associated Press reporter at the scene. Two NATO helicopters landed to airlift casualties while coalition troops using loudspeakers ordered bystanders to evacuate the area. Heavy black smoke poured from the bus, which was engulfed in a fireball.
Later, U.S. troops were seen carrying three black body bags from the bus' burned wreckage, which eyewitnesses said had been sandwiched in the convoy between mine-resistant armored coalition vehicles. The troops also were seen carrying a badly charred body from the bus.
The incident was reminiscent of another similar attack on a NATO convoy in May 2010. In that incident, a suicide bomber struck the convoy, killing 18 people, including five American troops and a Canadian. The Taliban, at the time, said the vehicle was packed with 1,650 pounds of explosives.
Earlier Saturday, a female suicide bomber blew herself up as she tried to attack a local government office in the capital of Kunar province, a hotbed of militancy in northeast Afghanistan along the Pakistan border.
Abdul Sabor Allayar, deputy provincial police chief, said the guards outside the government's intelligence office in Asad Abad became suspicious of the woman and started shooting, at which point she detonated her explosives.
There were no other casualties in that attack.
Afghan and U.S.-led coalition forces conducted operations earlier this month, killing more than 100 insurgents in an effort to curb violence in rugged areas of Kunar where the coalition and Afghan government have a light footprint.
Farther south along the Afghanistan-Pakistan border, Afghan and coalition forces captured two leaders of the Haqqani network and two other suspected insurgents in Sarobi district of Paktika province, the coalition said.
Haqqani fighters, who are affiliated with the Taliban and al Qaeda, are heavily rooted in Paktika and neighboring Paktia and Khost provinces.
One of the captured leaders provided insurgent fighters with funding, weapons, supplies and hideouts, and the other coordinated attacks against Afghan forces, the coalition said.