Three US soldiers and a military contractor were killed in a roadside explosion on Monday in the deadliest attack on American troops this year.
The deaths cast a shadow over continuing US peace talks with the Taliban, who claimed responsibility for the attack.
The blast, near Bagram Air Base, occurred as the US strives for peace in Afghanistan, more than 17 years after the Taliban were ousted.
The Nato-led Resolute Support mission in the country said the troops' convoy hit a roadside bomb that also wounded three soldiers.
Earlier, the Taliban had a different story, claiming one of its suicide bombers drove a vehicle filled with explosives into the convoy. They said "invaders" had been killed and that one armoured personnel carrier had been completely destroyed.
Bagram, America's largest air base in Afghanistan, is about 50 kilometres north of Kabul.
The explosion took place about 5.40pm local time, said Salim Noori, police spokesman for the province of Parwan.
Bagram district governor Abdul Shakoor Quddusi said a car bomber attacked an armoured vehicle carrying foreign forces close to a gate to the base.
Officials did not immediately release the names of those killed or the nationality of the contractor.
"The wounded service members were evacuated and are receiving medical care," Resolute Support said.
Democratic presidential hopeful Beto O'Rourke tweeted his condolences, saying: "Our thoughts are with the families of the three service members killed today in Afghanistan."
The blast brings to seven the number of US troops killed in Afghanistan so far this year, compared to 12 killed in 2018.
American casualties have fallen dramatically since the end of 2014, when Afghan forces took over from US-led Nato combat troops to secure the country.
The US now has about 14,000 troops in Afghanistan, down from a peak of around 100,000, most of whom are their as trainers and advisers for the Afghan soldiers.
Nearly 2,300 American troops have died and more than 20,400 have been wounded in the country since a US-led coalition ousted the Taliban in 2001.
While parts of Afghanistan including Kabul have had a lull in attacks in recent weeks, US and Afghan forces have stepped up attacks on the Taliban across the country.
Afghans are on high alert for a new round of violence when the militants kick off their expected spring offensive.
They typically declare a new fighting season as winter snow melts, and have in the past sought to gain control of district centres and target government centres.
Taliban leaders are set to meet Afghan officials next week for a new round of talks as the US tries to broker a peace deal.
Last week, US envoy Zalmay Khalilzad called for a "comprehensive and inclusive" Afghan dialogue including the government, women, youth and civil society.
The Taliban has until now refused to meet with the Kabul government, accusing it of being a puppet regime.
The diplomatic dispute between Washington and Kabul came to a head last month when Afghanistan's National Security Adviser Hamdullah Mohib accused Mr Khalilzad of a lack of "transparency", even suggesting the Afghan-born envoy wanted to be "viceroy" of his native country.
Washington reacted furiously, with US officials reportedly refusing to attend meetings at which Mr Mohib was present.
The outspoken adviser visited eastern provinces when Mr Khalilzad was in Kabul.
