Russia used Iran on August 16 for the first time as a base from which to launch air strikes against Syrian militants, widening its air campaign in Syria and deepening its involvement in the Middle East.
It was the first time Russia has used the territory of another nation, apart from Syria itself, to launch such strikes since the Kremlin launched a bombing campaign to support Syrian President Bashar al-Assad in September last year.
The Iranian deployment will boost Russia's image as a central player in the Middle East and allow the Russian air force to cut flight times and increase bombing payloads.
The head of Iran's National Security Council was quoted by state news agency IRNA as saying Tehran and Moscow were now sharing facilities.
Both countries back Assad. Russia, after a delay, has supplied Iran with its S-300 missile air defense system, evidence of a growing partnership that has helped turn the tide in Syria's civil war and is testing U.S. influence in the Middle East.
Relations between Tehran and Moscow have grown warmer since Iran reached agreement last year with global powers to curb its nuclear program in return for the lifting of U.N., EU and U.S. financial sanctions.
President Vladimir Putin visited in November and the two countries regularly discuss military planning for Syria. Iran has provided ground forces that work with local allies while Russia provides air power.
The United States said it was still assessing the extent of Russian-Iranian cooperation but described the new development as "unfortunate".
State Department spokesman Mark Toner said the United States was looking into whether the move violated UN Security Council resolution 2231, which prohibits the supply, sale and transfer of combat aircraft to Iran.
"It's unfortunate but not surprising," Toner told reporters. "It speaks to a continuation of a pattern we've seen of Russia continuing to carry out air strikes, now with Iran's direct assistance, ... that predominantly target moderate Syrian opposition forces."
He said U.S. Secretary of State John Kerry spoke by phone on August 16 to his Russian counterpart, Sergei Lavrov, who raised the issue.
Kerry is trying to reach agreement with Russia on military cooperation in the fight against Islamic State in Syria. Toner said those talks continued despite stepped up Russian-Iranian cooperation.
