The attack happened on the road to Kabul as a suicide car bomber blew up his vehicle and destroyed a bus carrying US troops.
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.
The security in the area where the attack occurred is under Afghan control. They don't seem to be making a lot of progress in pacifying the countryside.