President Barack Obama's top nuclear adviser said Tuesday that setbacks in Iran's uranium enrichment program have significantly delayed its progress toward building a nuclear weapon.
The president's coordinator for weapons of mass destruction, Gary Samore, also said that the United States had made clear to Russia that delivering a promised advanced air defense system to Iran would have serious implications on US-Russian relations.
Samore told reporters that he would be surprised if Russia fulfilled Tehran's 2007 order for the S-300 air defense system.
Russia has said that the order is on hold due to technical glitches, though many believe the delay stems from international opposition to the sale.
The S-300 missiles are capable of shooting down aircraft, cruise missiles and ballistic missile warheads at ranges of over 90 miles (145 kilometers) and at altitudes of about 90,000 feet. (27,500 meters)
On Iran's enrichment program, Samore said that Tehran had been set back by problems with its centrifuges and by disclosure of an enrichment plant near Qom that the United States alleges was part of a secret nuclear program.
Samore said that because of the setbacks, "the nuclear clock is not ticking as quickly as some had feared."
The United States and its allies are trying to rally support for new UN sanctions on Iran over its refusal to stop enrichment, fearing Tehran will use the process to build a nuclear weapon. Iran denies any intention to do so, saying its nuclear program aims only to generate electricity.