The Obama administration is exploring whether Russia's reported use of an Iranian air base to strike targets in Syria is a violation of a United Nations rules.

Several news outlets reported Tuesday that Russia struck targets in Syria from an Iranian air base, raising new questions about the ability of the U.S. and Russia to find some agreement in Syria and across the Middle East.

But it also raises questions about whether Russia is cooperating too closely with Iran under the UN resolution that approved the Iran nuclear agreement. In addition to the nuclear-related language, that resolution requires the UN to approve of any transfer of fighter aircraft to Iran, and the UN never approved Russia's use of Iran's air base.

"If these reports are true, it could very well be a violation of UN Security Council resolution 2231, which ... prohibits the supply, sale or transfer of combat aircraft to Iran unless approved in advance by the UN Security Council," State Department spokesman Mark Toner said.

Toner said that for now, it's not sure what might happen next if the U.S. determines Russia violated the resolution.

"I just don't have a definitive answer," he said. "I know our lawyers are kind of looking at ... and trying to collect as many details at this point."

But the issue is just the latest problem between the U.S. and Russia, which have tried unsuccessfully for months to find a way to cooperate in Syria. The U.S. says Russia continues to target some of the moderate Syrian groups that the U.S. is hoping to preserve in Syria's civil war.

Toner said Russia's attacked launched from Iran continue that trend of bombing groups the U.S. is supporting.

"I think it speaks to the continuation of a pattern that we've seen, of Russia continuing to carry out airstrikes, and now it appears with Iran's direct assistance, that at least purport to target ISIL and Daesh targets ... but in fact, and we've seen this continually, predominately target moderate Syrian opposition forces," Toner said.

"So that's unfortunate, and frankly that only makes more difficult what is already a very contentious and complex and difficult situation," he added.

Toner said the issue came up in a call between Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov. But he said he thinks Lavrov is the one who raised it, and that Kerry "stated our concerns."

Toner added that U.S. officials "just don't know" if Russia plans to continue using the Iranian air base, or if it was a one-off event.

"We don't know all the details about today's events and whether there is some kind of ongoing partnership or coordination effort with Iran," he said. "So I think we're still looking for clarification on that."