"Manas Transit Center contributes to the international effort to stabilize and secure Afghanistan and will only be used for that purpose," McFaul wrote.
Foreign Ministry spokesperson Alexander Lukashevich said Wednesday that it could not be ruled out that the U.S. would use the Manas base in a strike on Iran, Interfax reported.
Tensions have risen sharply in recent months over Iran's nuclear program, which the Islamic state claims is strictly for peaceful purposes. The U.S. and other Western powers have insisted on greater transparency in the program.