The Pentagon said on Tuesday it was concerned about the arms deal between Iran and Russia for missile systems.
"On the S-300, we've long expressed our concerns of reports on this possible sale of this missile system to the Iranians," said Pentagon spokesman Peter Cook at the daily briefing. "We don't see this as a positive development."
"At the same time, here at the Defense Department, again, the president will have options available to him even if that system is put into place," said Cook.
According to local media report, Iranian Defence Minister Hossein Dehghan announced on Tuesday an agreement had been reached with Russia for Tehran to procure the S-300 long-range surface-to-air missile systems.
"The text of the contract is ready and our friends will go to Russia next week to sign the contract," Dehghan was cited as saying by Iran's Fars News Agency.
The prospective arms sale came at a time when Iran and six world powers reached a deal in July to limit Tehran's nuclear program in exchange for lifting all nuclear-related sanctions.
The S-300 system, first deployed at the height of the Cold War in 1979, is designed for the air defense of large-scale facilities and can engage ballistic missiles up to 186 miles or about 300 kilometers away.