Afghan Interior Ministry spokesman Nasrat Rahimi says at least 42 people were wounded and 12 vehicles destroyed in the attack claimed by Afghan Taliban.
A Taliban suicide car bombing in Kabul on Thursday killed at least 10 Afghan civilians, a US service member and a Romanian soldier in a busy diplomatic area that includes the US Embassy.
It is the second such attack this week underscoring Afghan government warnings that a preliminary US-Taliban deal on ending America's longest war was moving dangerously quickly.
Interior Ministry spokesman Nasrat Rahimi said another 42 people were wounded and 12 vehicles destroyed in the explosion.
"Peace with a group that is still killing innocent people is meaningless," Afghan President Ashraf Ghani said in a statement.
A NATO Resolute Support mission statement said the two service members were "killed in action" without providing details or releasing their names pending notification of their families.
The American soldier was the fourth US service member killed in the past two weeks in Afghanistan.
Footage widely shared on social media showed the suicide bomber's vehicle turning into the checkpoint and exploding – and a passer-by trying to sprint away just seconds before.
"We all saw on security camera who were targeted," presidential spokesman Sediq Seddiqi tweeted.
Once again, stunned civilians were the victims, though US Ambassador John Bass in his condemnation of the attack said "members of security forces" were among the dead.
"I don't know who brought us to the hospital and how," said one of the wounded, Nezamuddin Khan, who was knocked unconscious and woke up in a local hospital.
The explosion at the checkpoint, which has been targeted in the past, follows a Taliban attack against a foreign compound late on Monday that killed at least 16 people and wounded more than 100, almost all of them local civilians.
Hours later, the Taliban claimed responsibility for a car bombing outside an Afghan military base in the Logar provincial capital, Puli Alam, which houses members of the Afghan special forces. Provincial council chief Haseebullah Stanekzai said a small number of international forces also had been in the area.
Governor Anwar Khan Es-Haqzai said four civilians were killed and four others wounded.
US-Taliban peace deal
US envoy Zalmay Khalilzad has been in Kabul this week briefing Ghani and other Afghan leaders on the US-Taliban deal to end nearly 18 years of fighting, which he says only needs US President Donald Trump's approval to become a reality.
Khalilzad has not commented publicly on this week's attacks.
The Afghan government has raised serious concerns about the deal, including in new comments on Thursday as the latest attack occurred.
The agreement was moving with "excessive speed", presidential adviser Waheed Omer told reporters, warning of difficult days ahead.
"Afghans have been bitten by this snake before," Omer said, recalling past agreements from which, like now, the Afghan government has been sidelined. "Where there is no feeling of ownership there is no safety," he said.
The Taliban, at their strongest since their 2001 defeat by a US-led invasion, has refused to negotiate with the government, calling it a US puppet.