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A car bomb killed two civilian contractors - reportedly both Americans - who were working in the Afghan capital on Monday, officials said.

"Two International Security Assistance Force contracted-civilians died as the result of a vehicle-borne improvised explosive device attack," the NATO-led International Security Assistance Force (ISAF) confirmed in a statement.

Speaking on condition of anonymity, a U.S. official told Reuters that the contractors were both American citizens. NBC News was not immediately able to verify the report.

The future of U.S. forces in the country remains uncertain as Washington has been unable to come to a agreement with Afghan President Hamid Karzai over a security agreement allowing some of its troops to remain beyond 2014.

Reuters contributed to this report.