Russia's decision to sell powerful surface-to-air missile systems to Iran will not thwart the Pentagon's plans for possible military action aimed at preventing Iran from building a nuclear weapon, the Defense Department's top officer said Thursday.

Army Gen. Martin Dempsey, chairman of the Joint Chiefs, said U.S. military planners were not surprised by Russia's announcement Monday that it would sell to Iran sophisticated weaponry that can potentially shoot down U.S. aircraft.

"We've known about the potential for that system to be sold to Iran for several years, and have accounted for it in all of our planning," Dempsey told reporters.

The U.S. and its allies are at a critical phase of negotiations with Iran about that nation's nuclear program. While they recently agreed on a "framework" that would halt the Iranian program, the details remain unresolved.

"The military option that I owe the president to both encourage the diplomatic solution, and if the diplomacy fails to ensure that Iran doesn't achieve a nuclear weapon, is intact," Dempsey said.

For years the U.S. has lobbied Russia to block the sale of the S-300 missile system to Iran for fear it could be used to defend Iranian nuclear sites from airstrikes. The missile system is also known as the SA-10 Grumble to the U.S. and its allies.

President Obama repeatedly has said he will consider military action to prevent Iran from obtaining an operational nuclear weapon. On April 2, Obama said failure to reach a deal to halt the nuclear program could "risk another war in the Middle East."

Some experts believe the deployment of S-300 missiles in Iran would make a military strike by the U.S. or Israel far more difficult.

Iran signed an $800 million deal with Russia to purchase the missiles in 2007, but Russia never followed through on the sale due to international pressures.

Defense Secretary Ash Carter said the Pentagon is not playing any direct role in the negotiations with Iran.

"Our role is not to conduct those negotiations, but two other things," Carter said at Thursday's briefing. "The first is to make sure that we have, as the president says, other options on the table. And that's something we take very seriously here and we do have other options on the table.

"And the second thing is to continue to play a stabilizing role in the region as a whole with all of our friends and allies, of which we have many there, and continue to strengthen their capabilities and their confidence. ... We're doing that."