The improvised bomb was detonated in the middle of the convoy about eight kilometres south of Asad Abad in eastern Kunar province, north of Kabul, according to Lt. Col. Douglas Lefforge.
“It was a deliberately planted bomb,” he said. But he added that he did not know who was behind the blast.
The US military have previously blamed similar attacks on remnants of the ousted Taliban regime and al-Qaeda organization that is blamed for the September 11 attacks on Washington and New York.
The spokesman described the condition of the soldiers as “stable”, saying they had been medically evacuated to Bagram Air Base.
If their injuries require further treatment, they would be transferred to Germany, Lefforge said.
Eight US-backed Afghan soldiers were killed and one seriously wounded on Friday in a similar incident in southeastern Khost province.
Afghan authorities said al-Qaeda fighters were behind the attack.
Last Friday, a US military base came under attack at Spin Boldak, east of Kandahar province, but there were no casualties.
“Two rockets impacted in the vicinity of the fire base at Spin Buldak last night,” he said.
Afghan police were searching for the attackers in a nearby village, Lefforge said.
There are around 11,500 US-led coalition troops in Afghanistan. About 9,000 of them are American.