The explosion in eastern Kabul was the latest incident to rattle the city ahead of April's presidential election due to choose the country's first new leader since 2001.
"Two International Security Assistance Force contracted civilians died as the result of a vehicle-borne improvised explosive device attack in eastern Afghanistan today," ISAF said in a statement.
A US official, speaking on condition of anonymity, said the slain contractors were American.
Afghanistan's future remains uncertain as the Taliban continue their insurgent campaign and Washington and President Hamid Karzai are deadlocked over a bilateral security deal to let some US forces stay beyond the end of 2014.
NATO forces have already begun to withdraw from Afghanistan, but the United States and other nations have been seeking to keep some troops in Afghanistan after 2014 to help the country's army fend off the Taliban.
On Saturday, the United Nations said that civilian deaths increased in 2013 as fighting intensified between government forces and insurgents.
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it is weird, there is no public backlash in america at the killings of these americans, it is like they are not even americans (compare that with the killings of americans in vietnam or on 911)