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WASHINGTON — President Barack Obama's top nuclear adviser says 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 U.S.-Russian relations.

Samore told reporters Tuesday that he would be surprised if Russia fulfilled Tehran's 2007 order for the S-300 air defense system.

Samore said that because of the setbacks in Iran's enrichment program, "the nuclear clock is not ticking as quickly as some had feared."