Another Romanian service member was also killed, a NATO-led coalition announced in a statement.
The attack does not penetrate the security perimeter, an official said.
Other details are not immediately available. The American's identity will be disclosed within 24 hours after the next relative is notified.
Halilzad said based on a draft agreement, the US will withdraw troops from five bases in Afghanistan within 135 days as long as the Taliban meet the conditions set out in the agreement.
The withdrawal of US troops could be the beginning of the end of America's longest war, a nearly 18-year conflict triggered by the September 11, 2001 terrorist attacks that cost billions of dollars to taxpayers and cost more than $ 2,300 life.