The United States voiced concern Monday about the possible sale of sophisticated Russian air defense missiles to Iran after Russian President Vladimir Putin lifted a ban on such shipments.
US Secretary of State John Kerry raised Washington's concerns about the end of the sales ban on the S-300 missile system directly with his Russian counterpart Sergei Lavrov in an early morning phone call.
The move comes just days after global powers -- Britain, China, France, Germany, Russia and the United States -- agreed an outline framework of a deal to sharply curtail Iran's suspect nuclear program.
But a full deal still has to be agreed by a June 30 deadline.
"We don't believe it's constructive at this time for Russia to move forward with it," said State Department acting spokeswoman Marie Harf.
The US military also expressed concerns. "Our opposition to these sales is long and public. We believe it's unhelpful," Pentagon spokesman Colonel Steve Warren told reporters.
Putin signed a decree lifting a self-imposed ban on supplying the missile system to the Islamic republic.
In doing so, he pre-empted the possible future lifting of sanctions against Iran if it agrees to the deal limiting its nuclear program.
"We think given Iran's destabilizing actions in the region, in places like Yemen or Syria or Lebanon, that this isn't the time to be selling these kinds of system to them," Harf told reporters.
But she conceded that the US administration did not believe that any such sales would be in violation of UN sanctions, slapped on Iran for its suspect nuclear program.
Asked whether sales would violate the UN Security Council sanctions, she replied: "It is my understanding, that it would not."
And she also stressed that the US has been working closely with Russia on the Iran nuclear negotiations, adding, "we don't think this will have an impact on unity in terms of the inside of the negotiating rooms."
While not the most sophisticated of Russia's missile systems, the S-300 would bolster Iran's defenses against any attack on its nuclear sites.
Neither Israel nor the United States have ruled out air strikes if Tehran pursues what Western powers fear is a bid to develop a nuclear bomb.
Moscow blocked a previous planned delivery of the surface-to-air missiles to Tehran in 2010 after the United Nations slapped sanctions on the Islamic republic.