The U.S. Embassy in Kabul provided little detail about the attack, but said that the two Americans were "lightly injured" and receiving medical treatment at a local hospital.
No one has yet claimed responsibility for the attack, which is currently under investigation by Afghan and U.S. authorities.
Wednesday's attack follows President Barack Obama's announcement on Tuesday that 9,800 U.S. troops will remain in Afghanistan beyond 2014, contingent upon Afghanistan's new leadership signing the bilateral security agreement with the U.S.
On May 23, four armed gunmen stormed the Indian consulate in the Herat province, after which Indian and Afghan security personnel killed them.
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