A top aide to President Obama forgave Iran's deployment of an advanced missile defense system around one of its nuclear facilities on Monday.

"It's not prohibited under any U.N. Security Council resolution because it's a defensive system," Obama national security adviser Ben Rhodes told reporters, after Iranian state media showed the deployment of the S-300 surface-to-air-missile system around Fordow, an underground uranium enrichment facility that stopped enrichment under the nuclear deal.

Russia restarted selling the missile system to Iran last April after halting a similar sale in 2010 due to Western pressure. Rhodes lauded the Kremlin's past willingness to cancel the transfer.

"This is something that the Russians helpfully delayed transfer for many years as we were negotiating the nuclear deal," he said.

Still, the administration's "strong preference" was that Iran not receive the S-300, Rhodes said.

"It's a defensive system, but its sophistication raised those concerns for us, and I know for some of our allies and partners in the region."

Soon after the renewed S-300 sale was announced, reports took note of the missile system's ability to make a strike on Iran's nuclear facilities—in the case that Tehran is not living up to its commitments—nearly impossible.

Rhodes, the Obama administration official credited with building an "echo chamber" to score support for the Iran nuclear deal, did not express concern about where the missiles were deployed.

"With respect to Fordow, if they're not enriching there, that addresses our immediate concern that it not be used as a facility where they can advance their nuclear capability," he said.

The Obama administration has not sanctioned the sale, despite being able to do so by law, and despite criticism from lawmakers.

"The Administration is failing to enforce U.S. laws that mandate sanctions against countries that export destabilizing advanced conventional weapons to Iran," Illinois senator Mark Kirk said in a statement Tuesday. "By allowing Iran to get destabilizing advanced S-300 missiles at Fordow, the Administration is making it harder to stop Iran from eventually getting nuclear weapons, especially because the flawed nuclear deal's temporary limits on Fordow's uranium enrichment capabilities completely expire in 15 years. That is, if we assume Iran does not cheat on the agreement first."